New module 'wmemcmp'.

New module 'wmemcmp'.

* modules/wmemcmp: New file.
* lib/wchar.in.h (wmemcmp): New declaration.
* lib/wmemcmp.c: New file.
* lib/wmemcmp-impl.h: New file, from libutf8 with modifications.
* m4/wmemcmp.m4: New file.
* m4/wchar_h.m4 (gl_WCHAR_H): Test whether wmemcmp is declared.
(gl_WCHAR_H_DEFAULTS): Initialize GNULIB_WMEMCMP, HAVE_WMEMCMP.
* modules/wchar (Makefile.am): Substitute GNULIB_WMEMCMP, HAVE_WMEMCMP.
* tests/test-wchar-c++.cc: Test the declaration of wmemcmp.
* doc/posix-functions/wmemcmp.texi: Mention the new module.