New module 'wcscat'.

New module 'wcscat'.

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