Fix compilation errors with clang that masquerades as gcc 13.
Reported by Sam James and Paul Eggert.
* m4/gnulib-common.m4 (gl_COMMON_BODY): For _GL_GNUC_PREREQ, ignore the
values of __GNUC__ and __GNUC_MINOR__ defined by clang.
* lib/warn-on-use.h (_GL_WARN_ON_USE, _GL_WARN_ON_USE_CXX): Don't treat
clang like GCC.
* lib/wchar.in.h (wmemchr, wcschr, wcsrchr, wcspbrk, wcsstr): Disable
the _GL_CXXALIASWARN1 and _GL_CXXALIASWARN invocations for masquerading
clang++.