wcscmp: Work around two ISO C compliance bugs on several platforms.
* lib/wchar.in.h (wcscmp): Consider REPLACE_WCSCMP.
* lib/wcscmp-impl.h (wcscmp): Don't assume that the two wide characters
are in the range 0..INT_MAX.
* m4/wcscmp.m4 (gl_FUNC_WCSCMP): Test whether wcscmp works for all wide
characters. Set REPLACE_WCSCMP.
* m4/wchar_h.m4 (gl_WCHAR_H_DEFAULTS): Initialize REPLACE_WCSCMP.
* modules/wchar (Makefile.am): Substitute REPLACE_WCSCMP.
* modules/wcscmp (Status, Notice): Un-obsolete this module.
(configure.ac): Consider REPLACE_WCSCMP.
* doc/posix-functions/wcscmp.texi: Mention the two bugs.