New module 'wcsncmp'.

New module 'wcsncmp'.

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