From 421acf0fd6e619d0d0a70518eddca08991f990c3 Mon Sep 17 00:00:00 2001
From: Marko Lindqvist <cazfi74@gmail.com>
Date: Sun, 29 Jan 2023 23:18:53 +0200
Subject: [PATCH 41/41] Update gnulib-common to serial 80 from gnulib upstream

See osdn #46592

Signed-off-by: Marko Lindqvist <cazfi74@gmail.com>
---
 dependencies/m4/gnulib-common.m4 | 403 ++++++++++++++++++++-----------
 1 file changed, 264 insertions(+), 139 deletions(-)

diff --git a/dependencies/m4/gnulib-common.m4 b/dependencies/m4/gnulib-common.m4
index 2db3376b01..c0181abdc5 100644
--- a/dependencies/m4/gnulib-common.m4
+++ b/dependencies/m4/gnulib-common.m4
@@ -1,4 +1,4 @@
-# gnulib-common.m4 serial 76
+# gnulib-common.m4 serial 80
 dnl Copyright (C) 2007-2023 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -38,6 +38,11 @@ AC_DEFUN([gl_COMMON_BODY], [
        AIX system header files and several gnulib header files use precisely
        this syntax with 'extern'.  */
 #  define _Noreturn [[noreturn]]
+# elif (defined __clang__ && __clang_major__ < 16 \
+        && defined _GL_WORK_AROUND_LLVM_BUG_59792)
+   /* Compile with -D_GL_WORK_AROUND_LLVM_BUG_59792 to work around
+      that rare LLVM bug, though you may get many false-alarm warnings.  */
+#  define _Noreturn
 # elif ((!defined __cplusplus || defined __clang__) \
         && (201112 <= (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0) \
             || (!defined __STRICT_ANSI__ \
@@ -119,29 +124,35 @@ AC_DEFUN([gl_COMMON_BODY], [
    by the Nth argument of the function is the size of the returned memory block.
  */
 /* Applies to: function, pointer to function, function types.  */
-#if _GL_HAS_ATTRIBUTE (alloc_size)
-# define _GL_ATTRIBUTE_ALLOC_SIZE(args) __attribute__ ((__alloc_size__ args))
-#else
-# define _GL_ATTRIBUTE_ALLOC_SIZE(args)
+#ifndef _GL_ATTRIBUTE_ALLOC_SIZE
+# if _GL_HAS_ATTRIBUTE (alloc_size)
+#  define _GL_ATTRIBUTE_ALLOC_SIZE(args) __attribute__ ((__alloc_size__ args))
+# else
+#  define _GL_ATTRIBUTE_ALLOC_SIZE(args)
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_ALWAYS_INLINE tells that the compiler should always inline the
    function and report an error if it cannot do so.  */
 /* Applies to: function.  */
-#if _GL_HAS_ATTRIBUTE (always_inline)
-# define _GL_ATTRIBUTE_ALWAYS_INLINE __attribute__ ((__always_inline__))
-#else
-# define _GL_ATTRIBUTE_ALWAYS_INLINE
+#ifndef _GL_ATTRIBUTE_ALWAYS_INLINE
+# if _GL_HAS_ATTRIBUTE (always_inline)
+#  define _GL_ATTRIBUTE_ALWAYS_INLINE __attribute__ ((__always_inline__))
+# else
+#  define _GL_ATTRIBUTE_ALWAYS_INLINE
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_ARTIFICIAL declares that the function is not important to show
     in stack traces when debugging.  The compiler should omit the function from
     stack traces.  */
 /* Applies to: function.  */
-#if _GL_HAS_ATTRIBUTE (artificial)
-# define _GL_ATTRIBUTE_ARTIFICIAL __attribute__ ((__artificial__))
-#else
-# define _GL_ATTRIBUTE_ARTIFICIAL
+#ifndef _GL_ATTRIBUTE_ARTIFICIAL
+# if _GL_HAS_ATTRIBUTE (artificial)
+#  define _GL_ATTRIBUTE_ARTIFICIAL __attribute__ ((__artificial__))
+# else
+#  define _GL_ATTRIBUTE_ARTIFICIAL
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_COLD declares that the function is rarely executed.  */
@@ -149,14 +160,16 @@ AC_DEFUN([gl_COMMON_BODY], [
 /* Avoid __attribute__ ((cold)) on MinGW; see thread starting at
    <https://lists.gnu.org/r/emacs-devel/2019-04/msg01152.html>.
    Also, Oracle Studio 12.6 requires 'cold' not '__cold__'.  */
-#if _GL_HAS_ATTRIBUTE (cold) && !defined __MINGW32__
-# ifndef __SUNPRO_C
-#  define _GL_ATTRIBUTE_COLD __attribute__ ((__cold__))
+#ifndef _GL_ATTRIBUTE_COLD
+# if _GL_HAS_ATTRIBUTE (cold) && !defined __MINGW32__
+#  ifndef __SUNPRO_C
+#   define _GL_ATTRIBUTE_COLD __attribute__ ((__cold__))
+#  else
+#   define _GL_ATTRIBUTE_COLD __attribute__ ((cold))
+#  endif
 # else
-#  define _GL_ATTRIBUTE_COLD __attribute__ ((cold))
+#  define _GL_ATTRIBUTE_COLD
 # endif
-#else
-# define _GL_ATTRIBUTE_COLD
 #endif
 
 /* _GL_ATTRIBUTE_CONST declares that it is OK for a compiler to omit duplicate
@@ -166,10 +179,12 @@ AC_DEFUN([gl_COMMON_BODY], [
    forever, and does not call longjmp.
    (This attribute is stricter than _GL_ATTRIBUTE_PURE.)  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (const)
-# define _GL_ATTRIBUTE_CONST __attribute__ ((__const__))
-#else
-# define _GL_ATTRIBUTE_CONST
+#ifndef _GL_ATTRIBUTE_CONST
+# if _GL_HAS_ATTRIBUTE (const)
+#  define _GL_ATTRIBUTE_CONST __attribute__ ((__const__))
+# else
+#  define _GL_ATTRIBUTE_CONST
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_DEALLOC (F, I) declares that the function returns pointers
@@ -178,10 +193,12 @@ AC_DEFUN([gl_COMMON_BODY], [
    _GL_ATTRIBUTE_DEALLOC_FREE declares that the function returns pointers that
    can be freed via 'free'; it can be used only after declaring 'free'.  */
 /* Applies to: functions.  Cannot be used on inline functions.  */
-#if _GL_GNUC_PREREQ (11, 0)
-# define _GL_ATTRIBUTE_DEALLOC(f, i) __attribute__ ((__malloc__ (f, i)))
-#else
-# define _GL_ATTRIBUTE_DEALLOC(f, i)
+#ifndef _GL_ATTRIBUTE_DEALLOC
+# if _GL_GNUC_PREREQ (11, 0)
+#  define _GL_ATTRIBUTE_DEALLOC(f, i) __attribute__ ((__malloc__ (f, i)))
+# else
+#  define _GL_ATTRIBUTE_DEALLOC(f, i)
+# endif
 #endif
 /* If gnulib's <string.h> or <wchar.h> has already defined this macro, continue
    to use this earlier definition, since <stdlib.h> may not have been included
@@ -205,16 +222,18 @@ AC_DEFUN([gl_COMMON_BODY], [
      - enumeration, enumeration item,
      - typedef,
    in C++ also: namespace, class, template specialization.  */
-#ifdef __has_c_attribute
-# if __has_c_attribute (__deprecated__)
-#  define _GL_ATTRIBUTE_DEPRECATED [[__deprecated__]]
-# endif
-#endif
-#if !defined _GL_ATTRIBUTE_DEPRECATED && _GL_HAS_ATTRIBUTE (deprecated)
-# define _GL_ATTRIBUTE_DEPRECATED __attribute__ ((__deprecated__))
-#endif
 #ifndef _GL_ATTRIBUTE_DEPRECATED
-# define _GL_ATTRIBUTE_DEPRECATED
+# ifdef __has_c_attribute
+#  if __has_c_attribute (__deprecated__)
+#   define _GL_ATTRIBUTE_DEPRECATED [[__deprecated__]]
+#  endif
+# endif
+# if !defined _GL_ATTRIBUTE_DEPRECATED && _GL_HAS_ATTRIBUTE (deprecated)
+#  define _GL_ATTRIBUTE_DEPRECATED __attribute__ ((__deprecated__))
+# endif
+# ifndef _GL_ATTRIBUTE_DEPRECATED
+#  define _GL_ATTRIBUTE_DEPRECATED
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_ERROR(msg) requests an error if a function is called and
@@ -222,24 +241,28 @@ AC_DEFUN([gl_COMMON_BODY], [
    _GL_ATTRIBUTE_WARNING(msg) requests a warning if a function is called and
    the function call is not optimized away.  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (error)
-# define _GL_ATTRIBUTE_ERROR(msg) __attribute__ ((__error__ (msg)))
-# define _GL_ATTRIBUTE_WARNING(msg) __attribute__ ((__warning__ (msg)))
-#elif _GL_HAS_ATTRIBUTE (diagnose_if)
-# define _GL_ATTRIBUTE_ERROR(msg) __attribute__ ((__diagnose_if__ (1, msg, "error")))
-# define _GL_ATTRIBUTE_WARNING(msg) __attribute__ ((__diagnose_if__ (1, msg, "warning")))
-#else
-# define _GL_ATTRIBUTE_ERROR(msg)
-# define _GL_ATTRIBUTE_WARNING(msg)
+#if !(defined _GL_ATTRIBUTE_ERROR && defined _GL_ATTRIBUTE_WARNING)
+# if _GL_HAS_ATTRIBUTE (error)
+#  define _GL_ATTRIBUTE_ERROR(msg) __attribute__ ((__error__ (msg)))
+#  define _GL_ATTRIBUTE_WARNING(msg) __attribute__ ((__warning__ (msg)))
+# elif _GL_HAS_ATTRIBUTE (diagnose_if)
+#  define _GL_ATTRIBUTE_ERROR(msg) __attribute__ ((__diagnose_if__ (1, msg, "error")))
+#  define _GL_ATTRIBUTE_WARNING(msg) __attribute__ ((__diagnose_if__ (1, msg, "warning")))
+# else
+#  define _GL_ATTRIBUTE_ERROR(msg)
+#  define _GL_ATTRIBUTE_WARNING(msg)
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_EXTERNALLY_VISIBLE declares that the entity should remain
    visible to debuggers etc., even with '-fwhole-program'.  */
 /* Applies to: functions, variables.  */
-#if _GL_HAS_ATTRIBUTE (externally_visible)
-# define _GL_ATTRIBUTE_EXTERNALLY_VISIBLE __attribute__ ((externally_visible))
-#else
-# define _GL_ATTRIBUTE_EXTERNALLY_VISIBLE
+#ifndef _GL_ATTRIBUTE_EXTERNALLY_VISIBLE
+# if _GL_HAS_ATTRIBUTE (externally_visible)
+#  define _GL_ATTRIBUTE_EXTERNALLY_VISIBLE __attribute__ ((externally_visible))
+# else
+#  define _GL_ATTRIBUTE_EXTERNALLY_VISIBLE
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_FALLTHROUGH declares that it is not a programming mistake if
@@ -247,16 +270,18 @@ AC_DEFUN([gl_COMMON_BODY], [
    'default' label.  The compiler should not warn in this case.  */
 /* Applies to: Empty statement (;), inside a 'switch' statement.  */
 /* Always expands to something.  */
-#ifdef __has_c_attribute
-# if __has_c_attribute (__fallthrough__)
-#  define _GL_ATTRIBUTE_FALLTHROUGH [[__fallthrough__]]
-# endif
-#endif
-#if !defined _GL_ATTRIBUTE_FALLTHROUGH && _GL_HAS_ATTRIBUTE (fallthrough)
-# define _GL_ATTRIBUTE_FALLTHROUGH __attribute__ ((__fallthrough__))
-#endif
 #ifndef _GL_ATTRIBUTE_FALLTHROUGH
-# define _GL_ATTRIBUTE_FALLTHROUGH ((void) 0)
+# ifdef __has_c_attribute
+#  if __has_c_attribute (__fallthrough__)
+#   define _GL_ATTRIBUTE_FALLTHROUGH [[__fallthrough__]]
+#  endif
+# endif
+# if !defined _GL_ATTRIBUTE_FALLTHROUGH && _GL_HAS_ATTRIBUTE (fallthrough)
+#  define _GL_ATTRIBUTE_FALLTHROUGH __attribute__ ((__fallthrough__))
+# endif
+# ifndef _GL_ATTRIBUTE_FALLTHROUGH
+#  define _GL_ATTRIBUTE_FALLTHROUGH ((void) 0)
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_FORMAT ((ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK))
@@ -270,10 +295,12 @@ AC_DEFUN([gl_COMMON_BODY], [
    If FIRST-TO-CHECK is not 0, arguments starting at FIRST-TO_CHECK
    are suitable for the format string.  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (format)
-# define _GL_ATTRIBUTE_FORMAT(spec) __attribute__ ((__format__ spec))
-#else
-# define _GL_ATTRIBUTE_FORMAT(spec)
+#ifndef _GL_ATTRIBUTE_FORMAT
+# if _GL_HAS_ATTRIBUTE (format)
+#  define _GL_ATTRIBUTE_FORMAT(spec) __attribute__ ((__format__ spec))
+# else
+#  define _GL_ATTRIBUTE_FORMAT(spec)
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_LEAF declares that if the function is called from some other
@@ -281,19 +308,23 @@ AC_DEFUN([gl_COMMON_BODY], [
    exception handling.  This declaration lets the compiler optimize that unit
    more aggressively.  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (leaf)
-# define _GL_ATTRIBUTE_LEAF __attribute__ ((__leaf__))
-#else
-# define _GL_ATTRIBUTE_LEAF
+#ifndef _GL_ATTRIBUTE_LEAF
+# if _GL_HAS_ATTRIBUTE (leaf)
+#  define _GL_ATTRIBUTE_LEAF __attribute__ ((__leaf__))
+# else
+#  define _GL_ATTRIBUTE_LEAF
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_MALLOC declares that the function returns a pointer to freshly
    allocated memory.  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (malloc)
-# define _GL_ATTRIBUTE_MALLOC __attribute__ ((__malloc__))
-#else
-# define _GL_ATTRIBUTE_MALLOC
+#ifndef _GL_ATTRIBUTE_MALLOC
+# if _GL_HAS_ATTRIBUTE (malloc)
+#  define _GL_ATTRIBUTE_MALLOC __attribute__ ((__malloc__))
+# else
+#  define _GL_ATTRIBUTE_MALLOC
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_MAY_ALIAS declares that pointers to the type may point to the
@@ -301,10 +332,12 @@ AC_DEFUN([gl_COMMON_BODY], [
    strict aliasing optimization.  */
 /* Applies to: types.  */
 /* Oracle Studio 12.6 mishandles may_alias despite __has_attribute OK.  */
-#if _GL_HAS_ATTRIBUTE (may_alias) && !defined __SUNPRO_C
-# define _GL_ATTRIBUTE_MAY_ALIAS __attribute__ ((__may_alias__))
-#else
-# define _GL_ATTRIBUTE_MAY_ALIAS
+#ifndef _GL_ATTRIBUTE_MAY_ALIAS
+# if _GL_HAS_ATTRIBUTE (may_alias) && !defined __SUNPRO_C
+#  define _GL_ATTRIBUTE_MAY_ALIAS __attribute__ ((__may_alias__))
+# else
+#  define _GL_ATTRIBUTE_MAY_ALIAS
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_MAYBE_UNUSED declares that it is not a programming mistake if
@@ -318,14 +351,22 @@ AC_DEFUN([gl_COMMON_BODY], [
    in C++ also: class.  */
 /* In C++ and C23, this is spelled [[__maybe_unused__]].
    GCC's syntax is __attribute__ ((__unused__)).
-   clang supports both syntaxes.  */
-#ifdef __has_c_attribute
-# if __has_c_attribute (__maybe_unused__)
-#  define _GL_ATTRIBUTE_MAYBE_UNUSED [[__maybe_unused__]]
-# endif
-#endif
+   clang supports both syntaxes.  Except that with clang ≥ 6, < 10, in C++ mode,
+   __has_c_attribute (__maybe_unused__) yields true but the use of
+   [[__maybe_unused__]] nevertheless produces a warning.  */
 #ifndef _GL_ATTRIBUTE_MAYBE_UNUSED
-# define _GL_ATTRIBUTE_MAYBE_UNUSED _GL_ATTRIBUTE_UNUSED
+# if defined __clang__ && defined __cplusplus
+#  if __clang_major__ >= 10
+#   define _GL_ATTRIBUTE_MAYBE_UNUSED [[__maybe_unused__]]
+#  endif
+# elif defined __has_c_attribute
+#  if __has_c_attribute (__maybe_unused__)
+#   define _GL_ATTRIBUTE_MAYBE_UNUSED [[__maybe_unused__]]
+#  endif
+# endif
+# ifndef _GL_ATTRIBUTE_MAYBE_UNUSED
+#  define _GL_ATTRIBUTE_MAYBE_UNUSED _GL_ATTRIBUTE_UNUSED
+# endif
 #endif
 /* Alternative spelling of this macro, for convenience and for
    compatibility with glibc/include/libc-symbols.h.  */
@@ -337,25 +378,38 @@ AC_DEFUN([gl_COMMON_BODY], [
    discard the return value.  The compiler may warn if the caller does not use
    the return value, unless the caller uses something like ignore_value.  */
 /* Applies to: function, enumeration, class.  */
-#ifdef __has_c_attribute
-# if __has_c_attribute (__nodiscard__)
-#  define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
-# endif
-#endif
-#if !defined _GL_ATTRIBUTE_NODISCARD && _GL_HAS_ATTRIBUTE (warn_unused_result)
-# define _GL_ATTRIBUTE_NODISCARD __attribute__ ((__warn_unused_result__))
-#endif
 #ifndef _GL_ATTRIBUTE_NODISCARD
-# define _GL_ATTRIBUTE_NODISCARD
+# if defined __clang__ && defined __cplusplus
+  /* With clang up to 15.0.6 (at least), in C++ mode, [[__nodiscard__]] produces
+     a warning.
+     The 1000 below means a yet unknown threshold.  When clang++ version X
+     starts supporting [[__nodiscard__]] without warning about it, you can
+     replace the 1000 with X.  */
+#  if __clang_major__ >= 1000
+#   define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
+#  endif
+# elif defined __has_c_attribute
+#  if __has_c_attribute (__nodiscard__)
+#   define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
+#  endif
+# endif
+# if !defined _GL_ATTRIBUTE_NODISCARD && _GL_HAS_ATTRIBUTE (warn_unused_result)
+#  define _GL_ATTRIBUTE_NODISCARD __attribute__ ((__warn_unused_result__))
+# endif
+# ifndef _GL_ATTRIBUTE_NODISCARD
+#  define _GL_ATTRIBUTE_NODISCARD
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_NOINLINE tells that the compiler should not inline the
    function.  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (noinline)
-# define _GL_ATTRIBUTE_NOINLINE __attribute__ ((__noinline__))
-#else
-# define _GL_ATTRIBUTE_NOINLINE
+#ifndef _GL_ATTRIBUTE_NOINLINE
+# if _GL_HAS_ATTRIBUTE (noinline)
+#  define _GL_ATTRIBUTE_NOINLINE __attribute__ ((__noinline__))
+# else
+#  define _GL_ATTRIBUTE_NOINLINE
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_NONNULL ((N1, N2,...)) declares that the arguments N1, N2,...
@@ -363,20 +417,24 @@ AC_DEFUN([gl_COMMON_BODY], [
    _GL_ATTRIBUTE_NONNULL () declares that all pointer arguments must not be
    null.  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (nonnull)
-# define _GL_ATTRIBUTE_NONNULL(args) __attribute__ ((__nonnull__ args))
-#else
-# define _GL_ATTRIBUTE_NONNULL(args)
+#ifndef _GL_ATTRIBUTE_NONNULL
+# if _GL_HAS_ATTRIBUTE (nonnull)
+#  define _GL_ATTRIBUTE_NONNULL(args) __attribute__ ((__nonnull__ args))
+# else
+#  define _GL_ATTRIBUTE_NONNULL(args)
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_NONSTRING declares that the contents of a character array is
    not meant to be NUL-terminated.  */
 /* Applies to: struct/union members and variables that are arrays of element
    type '[[un]signed] char'.  */
-#if _GL_HAS_ATTRIBUTE (nonstring)
-# define _GL_ATTRIBUTE_NONSTRING __attribute__ ((__nonstring__))
-#else
-# define _GL_ATTRIBUTE_NONSTRING
+#ifndef _GL_ATTRIBUTE_NONSTRING
+# if _GL_HAS_ATTRIBUTE (nonstring)
+#  define _GL_ATTRIBUTE_NONSTRING __attribute__ ((__nonstring__))
+# else
+#  define _GL_ATTRIBUTE_NONSTRING
+# endif
 #endif
 
 /* There is no _GL_ATTRIBUTE_NORETURN; use _Noreturn instead.  */
@@ -384,10 +442,12 @@ AC_DEFUN([gl_COMMON_BODY], [
 /* _GL_ATTRIBUTE_NOTHROW declares that the function does not throw exceptions.
  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (nothrow) && !defined __cplusplus
-# define _GL_ATTRIBUTE_NOTHROW __attribute__ ((__nothrow__))
-#else
-# define _GL_ATTRIBUTE_NOTHROW
+#ifndef _GL_ATTRIBUTE_NOTHROW
+# if _GL_HAS_ATTRIBUTE (nothrow) && !defined __cplusplus
+#  define _GL_ATTRIBUTE_NOTHROW __attribute__ ((__nothrow__))
+# else
+#  define _GL_ATTRIBUTE_NOTHROW
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_PACKED declares:
@@ -396,10 +456,12 @@ AC_DEFUN([gl_COMMON_BODY], [
    minimizing the memory required.  */
 /* Applies to: struct members, struct, union,
    in C++ also: class.  */
-#if _GL_HAS_ATTRIBUTE (packed)
-# define _GL_ATTRIBUTE_PACKED __attribute__ ((__packed__))
-#else
-# define _GL_ATTRIBUTE_PACKED
+#ifndef _GL_ATTRIBUTE_PACKED
+# if _GL_HAS_ATTRIBUTE (packed)
+#  define _GL_ATTRIBUTE_PACKED __attribute__ ((__packed__))
+# else
+#  define _GL_ATTRIBUTE_PACKED
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_PURE declares that It is OK for a compiler to omit duplicate
@@ -409,19 +471,23 @@ AC_DEFUN([gl_COMMON_BODY], [
    observable state, and always returns exactly once.
    (This attribute is looser than _GL_ATTRIBUTE_CONST.)  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (pure)
-# define _GL_ATTRIBUTE_PURE __attribute__ ((__pure__))
-#else
-# define _GL_ATTRIBUTE_PURE
+#ifndef _GL_ATTRIBUTE_PURE
+# if _GL_HAS_ATTRIBUTE (pure)
+#  define _GL_ATTRIBUTE_PURE __attribute__ ((__pure__))
+# else
+#  define _GL_ATTRIBUTE_PURE
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_RETURNS_NONNULL declares that the function's return value is
    a non-NULL pointer.  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (returns_nonnull)
-# define _GL_ATTRIBUTE_RETURNS_NONNULL __attribute__ ((__returns_nonnull__))
-#else
-# define _GL_ATTRIBUTE_RETURNS_NONNULL
+#ifndef _GL_ATTRIBUTE_RETURNS_NONNULL
+# if _GL_HAS_ATTRIBUTE (returns_nonnull)
+#  define _GL_ATTRIBUTE_RETURNS_NONNULL __attribute__ ((__returns_nonnull__))
+# else
+#  define _GL_ATTRIBUTE_RETURNS_NONNULL
+# endif
 #endif
 
 /* _GL_ATTRIBUTE_SENTINEL(pos) declares that the variadic function expects a
@@ -429,17 +495,21 @@ AC_DEFUN([gl_COMMON_BODY], [
    _GL_ATTRIBUTE_SENTINEL () - The last argument is NULL (requires C99).
    _GL_ATTRIBUTE_SENTINEL ((N)) - The (N+1)st argument from the end is NULL.  */
 /* Applies to: functions.  */
-#if _GL_HAS_ATTRIBUTE (sentinel)
-# define _GL_ATTRIBUTE_SENTINEL(pos) __attribute__ ((__sentinel__ pos))
-#else
-# define _GL_ATTRIBUTE_SENTINEL(pos)
+#ifndef _GL_ATTRIBUTE_SENTINEL
+# if _GL_HAS_ATTRIBUTE (sentinel)
+#  define _GL_ATTRIBUTE_SENTINEL(pos) __attribute__ ((__sentinel__ pos))
+# else
+#  define _GL_ATTRIBUTE_SENTINEL(pos)
+# endif
 #endif
 
 /* A helper macro.  Don't use it directly.  */
-#if _GL_HAS_ATTRIBUTE (unused)
-# define _GL_ATTRIBUTE_UNUSED __attribute__ ((__unused__))
-#else
-# define _GL_ATTRIBUTE_UNUSED
+#ifndef _GL_ATTRIBUTE_UNUSED
+# if _GL_HAS_ATTRIBUTE (unused)
+#  define _GL_ATTRIBUTE_UNUSED __attribute__ ((__unused__))
+# else
+#  define _GL_ATTRIBUTE_UNUSED
+# endif
 #endif
 
 ]dnl There is no _GL_ATTRIBUTE_VISIBILITY; see m4/visibility.m4 instead.
@@ -450,10 +520,12 @@ AC_DEFUN([gl_COMMON_BODY], [
 /* Applies to: label (both in C and C++).  */
 /* Note that g++ < 4.5 does not support the '__attribute__ ((__unused__)) ;'
    syntax.  But clang does.  */
-#if !(defined __cplusplus && !_GL_GNUC_PREREQ (4, 5)) || defined __clang__
-# define _GL_UNUSED_LABEL _GL_ATTRIBUTE_UNUSED
-#else
-# define _GL_UNUSED_LABEL
+#ifndef _GL_UNUSED_LABEL
+# if !(defined __cplusplus && !_GL_GNUC_PREREQ (4, 5)) || defined __clang__
+#  define _GL_UNUSED_LABEL _GL_ATTRIBUTE_UNUSED
+# else
+#  define _GL_UNUSED_LABEL
+# endif
 #endif
 ])
   AH_VERBATIM([async_safe],
@@ -1026,8 +1098,21 @@ AC_DEFUN([gl_CONDITIONAL_HEADER],
 dnl gl_CHECK_FUNCS_ANDROID([func], [[#include <foo.h>]])
 dnl is like AC_CHECK_FUNCS([func]), taking into account a portability problem
 dnl on Android.
-dnl Namely, if func was added to Android API level, say, 28, then the libc.so
-dnl has the symbol func always, whereas the header file <foo.h> declares func
+dnl
+dnl When code is compiled on Android, it is in the context of a certain
+dnl "Android API level", which indicates the minimum version of Android on
+dnl which the app can be installed. In other words, you don't compile for a
+dnl specific version of Android. You compile for all versions of Android,
+dnl onwards from the given API level.
+dnl Thus, the question "does the OS have the function func" has three possible
+dnl answers:
+dnl   - yes, in all versions starting from the given API level,
+dnl   - no, in no version,
+dnl   - not in the given API level, but in a later version of Android.
+dnl
+dnl In detail, this works as follows:
+dnl If func was added to Android API level, say, 28, then the libc.so has the
+dnl symbol func always, whereas the header file <foo.h> declares func
 dnl conditionally:
 dnl   #if __ANDROID_API__ >= 28
 dnl   ... func (...) __INTRODUCED_IN(28);
@@ -1035,15 +1120,55 @@ dnl   #endif
 dnl Thus, when compiling with "clang -target armv7a-unknown-linux-android28",
 dnl the function func is declared and exists in libc.
 dnl Whereas when compiling with "clang -target armv7a-unknown-linux-android27",
-dnl the function func is not declared but exists in libc. We need to treat this
-dnl case like the case where func does not exist.
+dnl the function func is not declared but exists in libc.
+dnl
+dnl This macro sets two variables:
+dnl   - gl_cv_onwards_func_<func>   to yes / no / "future OS version"
+dnl   - ac_cv_func_<func>           to yes / no / no
+dnl The first variable allows to distinguish all three cases.
+dnl The second variable is set, so that an invocation
+dnl   gl_CHECK_FUNCS_ANDROID([func], [[#include <foo.h>]])
+dnl can be used as a drop-in replacement for
+dnl   AC_CHECK_FUNCS([func]).
 AC_DEFUN([gl_CHECK_FUNCS_ANDROID],
 [
-  AC_CHECK_DECL([$1], , , [$2])
-  if test $ac_cv_have_decl_[$1] = yes; then
-    AC_CHECK_FUNCS([$1])
-  else
-    ac_cv_func_[$1]=no
+  AC_REQUIRE([AC_CANONICAL_HOST])
+  AC_CACHE_CHECK([for [$1]],
+    [[gl_cv_onwards_func_][$1]],
+    [gl_SILENT([
+       case "$host_os" in
+         linux*-android*)
+           AC_CHECK_DECL([$1], , , [$2])
+           if test $[ac_cv_have_decl_][$1] = yes; then
+             AC_CHECK_FUNC([[$1]])
+             if test $[ac_cv_func_][$1] = yes; then
+               [gl_cv_onwards_func_][$1]=yes
+             else
+               dnl The function is declared but does not exist. This should not
+               dnl happen normally. But anyway, we know that a future version
+               dnl of Android will have the function.
+               [gl_cv_onwards_func_][$1]='future OS version'
+             fi
+           else
+             [gl_cv_onwards_func_][$1]='future OS version'
+           fi
+           ;;
+         *)
+           AC_CHECK_FUNC([$1])
+           [gl_cv_onwards_func_][$1]=$[ac_cv_func_][$1]
+           ;;
+       esac
+      ])
+    ])
+  case "$[gl_cv_onwards_func_][$1]" in
+    future*) [ac_cv_func_][$1]=no ;;
+    *)       [ac_cv_func_][$1]=$[gl_cv_onwards_func_][$1] ;;
+  esac
+  if test $[ac_cv_func_][$1] = yes; then
+    AC_DEFINE([HAVE_]m4_translit([[$1]],
+                                 [abcdefghijklmnopqrstuvwxyz],
+                                 [ABCDEFGHIJKLMNOPQRSTUVWXYZ]),
+              [1], [Define to 1 if you have the `$1' function.])
   fi
 ])
 
-- 
2.39.0