New module 'wcsncasecmp'.

New module 'wcsncasecmp'.

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