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