wctype-h: When overriding wint_t, override also the related functions.
Reported by Christian Biesinger <cbiesinger@google.com> in
<https://lists.gnu.org/archive/html/bug-gnulib/2019-11/msg00027.html>.
* m4/wctype_h.m4 (gl_WCTYPE_H): When GNULIB_OVERRIDES_WINT_T is 1, set
REPLACE_ISWCNTRL to 1.
* lib/wctype.in.h (rpl_iswalnum, rpl_iswalpha, rpl_iswblank,
rpl_iswcntrl, rpl_iswdigit, rpl_iswgraph, rpl_iswlower, rpl_iswprint,
rpl_iswpunct, rpl_iswspace, rpl_iswupper, rpl_iswxdigit, rpl_towlower,
rpl_towupper): New definitions when GNULIB_OVERRIDES_WINT_T is 1.
* doc/posix-headers/wchar.texi: Mention that wint_t is also overridden
on mingw.
* doc/posix-headers/wctype.texi: Likewise.