New module 'wcsxfrm'.

New module 'wcsxfrm'.

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