New module 'wcschr'.

New module 'wcschr'.

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