New module 'wcscmp'.

New module 'wcscmp'.

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