ffmpeg update, and bundling in ffmpeg, for MinGW and Linux

This puts ffmpeg sources in-tree and integrates our autoconf/automake with ffmpeg's build process. The default and official way to do things will now be to use the included ffmpeg in lieu of anything present on the system. There's just too many versions (plus the ffmpeg/libav convergence) and they're too different for us to be trying to support people using whatever their distro shipped with anymore.

Pullreqing because this is a big change and I'd like some eyes on it to catch mistakes before I break the build.

Also doing away with build.sh, because this replaces the only useful thing build.sh did. (You can make a GameData.smzip by just running "make SMData" after configuring.)