New module 'wmemcpy'.

New module 'wmemcpy'.

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