Uses dlopen to load AvxSynth on Linux and OS X.

Allows the use of --demuxer avs for AvxSynth, though the only source filter it
can currently use is FFMS2.

Add a local copy of avxsynth_c.h and its dependent headers in extras/ so that
users don't need to actually have AvxSynth development headers installed to
enable support for it (mirroring the AviSynth behavior).

Based on a patch by 0x09 (tab@lavabit.com)