New module 'wcstok'.

New module 'wcstok'.

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