ustring: Fix wchar conversion on macOS with libc++

libc++ internally represents std::wstrings in UCS-4 just like libstdc++,
but does not set the __STDC_ISO_10646__ define. This caused the code to
fall back to calling iconv with the WCHAR_T source character set, which
does not correctly convert these strings and leads to errors, for
example in inkscape on startup.

See https://trac.macports.org/ticket/56214 for an instance of such
a problem.

Re-use the UCS4 to UTF8 conversion code when libc++ is detected, which
solves this. Bug 795338