Add character set related __STDC_* definitions.

Clang uses UTF-16 and UTF-32 for its char16_t's and char32_t's
exclusively. This means that we can define __STDC_UTF_16__ and
__STDC_UTF_32__ unconditionally.

While there, define __STDC_MB_MIGHT_NEQ_WC__ for FreeBSD. FreeBSD's
wchar_t's don't encode characters as ISO-10646; the encoding depends on
the locale used. Because the character set used might not be a superset
of ASCII, we must define __STDC_MB_MIGHT_NEQ_WC__.