New module 'wcsstr'.

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