New module 'wcsrchr'.

New module 'wcsrchr'.

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