Fix bug reported by Bruno Haible in

Fix bug reported by Bruno Haible in
<http://lists.gnu.org/archive/html/bug-gnulib/2006-12/msg00228.html>
where quotearg.c didn't compile on Mac OS X 10.2 because it
lacks <wchar.h> and wint_t.
* lib/wctype_.h (__wctype_wint_t): New type.
Include <stdio.h>, <time.h>, <wchar.h> only if HAVE_WINT_T.
(iswalnum, iswalpha, iswblank, iswcntrl, iswdigit, iswgraph):
(iswlower, iswprint, iswpunct, iswspace, iswupper, ixwxdigit):
Arg is now of type __wctype_wint_t, not wint_t.
* m4/wctype.m4 (gl_WCTYPE_H): Require gt_TYPE_WINT_T, and
substitute HAVE_WINT_T.
* modules/wctype (Files): Add m4/wint_t.m4.
(wctype.h): Substitute HAVE_WINT_T.