Further reduce ifdeffing.

Also make the avs demuxer work with AvxSynth.  Due to the lack of
VfW, DirectShow, and DG* tools on Linux/OSX, the only one that it
can use is FFMS2.
Don't link to AvxSynth, just query the existence of avxsynth_c.h
Clean up the commented-out parts of configure's AvxSynth detection