Avoid clang warnings regarding [[__maybe_unused__]]

[Bruno Haible]

Building a Gnulib testdir on FreeBSD 12.0, which uses clang 6.0.1, I see
many warnings such as

../../gltests/test-dirent-c++.cc:28:1: warning: unknown attribute 
'__maybe_unused__' ignored [-Wunknown-attributes]

The cause is that in this version of clang, __has_c_attribute(__maybe_unused__)
apparently returns true, and the use of [[__maybe_unused__]] does not elicit a
syntax error. But it elicits a warning. Here's how it depends on the clang
version:

clang   result
-----   -------
4.0.0   error
5.0.2   error
6.0.1   warning
7.0.1   warning
8.0.0   warning
9.0.0   warning
10.0.0  ok

This patch fixes the warnings, by choosing a different (more conservative)
expansion of _GL_ATTRIBUTE_MAYBE_UNUSED.


2023-01-28  Bruno Haible  <bruno@clisp.org>

        Avoid clang warnings regarding [[__maybe_unused__]].
        * m4/gnulib-common.m4 (gl_COMMON_BODY): For clang versions ≥ 6, < 10,
        in C++ mode, ignore the __has_c_attribute value and define
        _GL_ATTRIBUTE_MAYBE_UNUSED to __attribute__ ((__unused__)), not
        [[__maybe_unused__]].

diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index 0f1a6ac1ea..cf0fc5a1e7 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -1,4 +1,4 @@
-# gnulib-common.m4 serial 78
+# gnulib-common.m4 serial 79
 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,
@@ -351,9 +351,15 @@ 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.  */
+   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
-# ifdef __has_c_attribute
+# 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