New module 'wcsspn'.

New module 'wcsspn'.

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