New module 'wcscoll'.

New module 'wcscoll'.

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