Fix wint_t on MSVC. * lib/wchar.in.h (wint_t): On MSVC, override it. * lib/wctype.in.h (wint_t): Likewise. * m4/stdint.m4 (gl_STDINT_TYPE_PROPERTIES): Override BITSIZEOF_WINT_T on MSVC. * doc/posix-headers/wchar.texi: Mention the problem with wint_t on MSVC. * doc/posix-headers/wctype.texi: Likewise.