New module 'wcspbrk'.

New module 'wcspbrk'.

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