AviSynth+ now supports non-Windows OSes, making AvxSynth
obsolete.  Since we no longer support AviSynth 2.5 (which is
essentially what AvxSynth is), remove AvxSynth support and
replace it with AviSynth+.

As a result, the USING_AVISYNTH defines can be switched back
to generic _WIN32.