wmemcmp: Work around ISO C compliance bug on several platforms.
* lib/wchar.in.h (wmemcmp): Consider REPLACE_WMEMCMP.
* lib/wmemcmp-impl.h (wmemcmp): Don't assume that the two wide
characters are in the range 0..INT_MAX.
* m4/wmemcmp.m4 (gl_FUNC_WMEMCMP): Test whether wmemcmp works for all
wide characters. Set REPLACE_WMEMCMP.
* m4/wchar_h.m4 (gl_WCHAR_H_DEFAULTS): Initialize REPLACE_WMEMCMP.
* modules/wchar (Makefile.am): Substitute REPLACE_WMEMCMP.
* modules/wmemcmp (configure.ac): Consider REPLACE_WMEMCMP.
* doc/posix-functions/wmemcmp.texi: Mention the bug.