AvxSynth-based script input support for non-Windows platforms

Patch by 0x09 (tab@lavabit.com)

A vanilla AvxSynth install has an issue with where it looks for headers (somewhat described in issue #70: https://github.com/avxsynth/avxsynth/issues/70), but if the user adjusts that on AvxSynth's end, then this patch works correctly.