New module 'wcpcpy'.

New module 'wcpcpy'.

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