In !863 I assumed that a 4 byte wchar_t would be equal to gunichar on all platforms,
but this is not the case for FreeBSD. It previously was using the non wchar_t based API
because it doesn't define __STDC_ISO_10646__.

Add back the __STDC_ISO_10646__ check again and introduce a more meaningful macro for guarding
the code paths that are mixing gunichar and wchar_t.

Fixes #1798