wchar: Fix compilation error with the original mingw.org mingw.
* lib/wchar.in.h (rpl_wint_t): If <crtdefs.h> does not exist, include
<stddef.h> instead.
* m4/wint_t.m4 (gl_TYPE_WINT_T_PREREQ): New macro, extracted from
gl_WCTYPE_H.
* m4/wctype_h.m4 (gl_WCTYPE_H): Don't set HAVE_CRTDEFS_H here; require
gl_TYPE_WINT_T_PREREQ instead.
* m4/wchar_h.m4 (gl_WCHAR_H): Require gl_TYPE_WINT_T_PREREQ.
* modules/wchar (Makefile.am): Substitute HAVE_CRTDEFS_H.
Reported by Eli Zaretskii <eliz@gnu.org>.