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.
Fix compilation since AVCODEC_MAX_AUDIO_FRAME_SIZE was removed.

Using deprecated macro, but still better than broken.
Fix compilation with --disable-audio
Use liblsmash for L-SMASH-based mp4 support instead of the internal copy in output/mp4.

The internal copy has been removed.

The only caveat is that there's still a need for importer.h from
L-SMASH's source tree, and since this header is not installed with
L-SMASH, the user is required to manually copy it over themselves.
Since 'importer' is a bit of a generic name, the source files that
require it will be looking for lsmash_importer.h.

This could of course be avoided by patching L-SMASH to install its
headers to a subdirectory in the install path and put both lsmash.h
and importer.h there.  That's outside the scope of this patch, though.
Fix compilation against latest L-SMASH git HEAD.
configure: Add --disable-mp3 option
always use strict=-2 for aac

FFmpeg internal aac encoder needs this.
assume channels = 2 when can't get them
Add --chpl-with-bom option
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