Add sequential revision number to versioning
AviSynth script support that relies on a discrete demuxer that loads AvxSynth, rather than libavformat or the removed win32 binary loader.

Only works on Linux and OSX. Use --disable-avxsynth for Windows
builds. libavformat's AviSynth demuxer can be used on Windows,
which negates the need for this on there anyway.

Patch by 0x09 (tab@lavabit.com)
demux_mkv: Support playing Opus streams in Matroska

FFmpeg recently changed how it writes Opus-in-Matroska to match
the A_OPUS/EXPERIMENTAL name that mkvmerge uses, with the caveat
that things will change and compatibility with old files can get
worked out when the spec is finalized.

This adds both A_OPUS and A_OPUS/EXPERIMENTAL so that *hopefully*
it can play both the newer files that use A_OPUS/EXPERIMENTAL, and
older ones muxed by FFmpeg that were simply A_OPUS, since this is
also what FFmpeg seems to be doing to handle the situation.
Make AvxSynth properly detectable so that --disable-avxsynth isn't
needed for Windows builds.