Use blessed-by-draft-standard way to test for [[__deprecated__]]
etc.; this is supported by GCC 11 and the fallback code should
work fine with GCC 10 and earlier, clang, etc.
* NEWS, doc/attribute.texi:
Mention that DEPRECATED etc. must now come first.
* lib/fflush.c, lib/unistr.in.h:
Do not include unused-parameter.h. All uses of _GL_UNUSED_PARAMETER
(after parameters) replaced by _GL_ATTRIBUTE_MAYBE_UNUSED (before
parameters). Although perhaps we need a shorter name for
_GL_ATTRIBUTE_MAYBE_UNUSED, that should probably be in
gnulib-common.m4, due to the ubiquity of this issue.
And perhaps the snippet/unused-parameter module should be marked
obsolete since it's no longer compatible with its old use.
* m4/gnulib-common.m4 (gl_COMMON_BODY): Define the macro
_GL_HAS_C_ATTRIBUTE, and use it instead of __STDC_VERSION__ in
deciding whether to use C2x attributes like [[__deprecated__]].
(_GL_ATTRIBUTE_MAYBE_UNUSED): Fall back on _GL_ATTRIBUTE_UNUSED.
(_GL_ATTRIBUTE_UNUSED): New macro, for labels where C2x
[[maybe_unused]] do not work.
(_GL_UNUSED, _GL_UNUSED_LABEL): Use it.
* modules/fflush, modules/unistr/base (Depends-on):
Remove snippet/unused-parameter.
* modules/fflush (selinux/selinux.h, selinux/context.h)
(selinux/label.h): Do not use $(UNUSED_PARAMETER).
Remove useless chmod a-x.