New module 'wcscasecmp'.

New module 'wcscasecmp'.

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