Assorted fixes

Fixing compilation against FFMS2-svn by entirely removing postproc from avxffms2.  Hinges on if/when a decision is made to support that option rather than relying solely on point releases for FFmpeg and FFMS2 (as current FFmpeg can no longer build with FFMS2 2.17).

The autotools gitignore stuff can be ignored if not desired.  It's there for convenience only.

The header stuff makes sure all includes are consistent, and refer to the proper location without the user needing to pass extra parameters to a software project that uses it.

And a small change that silences deprecation warnings in avxffms2.