Get audio from AvxSynth as well.

Based on a patch by 0x09 (tab@lavabit.com). The original patch
description as follows:

In this case since they both target 2.5, the only modifications
to input/audio/avs.c are linking av[ix]synth instead of loading,
and not being usable as an importer for regular audio files (only
scripts)*.

*The other one allows things like

--audiofile somefile.mp3 --ademuxer avs

and opens somefile with (e.g.) DirectShowSource. Since all we have
is ffms2 it's kind of pointless.
More detailed version head
Fix compilation with --disable-audio
Skip useless lavr check when lavf is not available
Add an option to use r2204 style progress indicator
Add back "touhou" tune
Film Grain Optimization
Fade Compensation
Remove stats before renaming
Add a parameter to set level of writing options in User Data Unregistered SEI
Utilize internal threading in lavf/ffms
Auto VBV settings
AVI output
filters hqdn3d pad vflip yadif patch
Log file
Encoding time
Level force
Profile force
Cosmetic
New experimental AQ mode (modification of Auto-variance AQ)