New module 'wcscspn'.

New module 'wcscspn'.

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