New module 'wcswidth'.

New module 'wcswidth'.

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