See the mailing list thread <https://lists.fedoraproject.org/archives/list/
devel@lists.fedoraproject.org/thread/SZ676IHHSLOQD6UN2I5J5VKXJ5P5SOVO/>
"glib-2.0 G_CONST_RETURN causing GCC 'warning: const' on F31", where the GCC
diagnostic

> test.c:2:13: warning: const
>     2 | G_CONST_RETURN char * f();
>       |             ^~~~~~~

had confused me, and "Deprecated pre-processor symbol, repace with const" is
probably a better warning message than just "const".

(That recent GCC only prints "Deprecated pre-processor symbol, repace with "
appears to be a bug in GCC that GLIB_UNAVAILABLE_MACRO already suffers from,
too.  Recent Clang correctly prints "Deprecated pre-processor symbol, repace
with const".)