CMake checks for FFmpeg API changes
FFmpeg breaks bc every so often. The safest way to deal with this is
with CMake checks for the new API. This implements such checks for most
of the recent changes and deprecations (one function that is deprecated
in ffmpeg 0.11 remains).
This simplifies the code considerably, at the expense of complicating CMakeLists.txt a little.
There is one remaining deprecated function: avcode_decode_audio3, which is replaced by avcode_decode_audio4. However, this produces output in quite a different format, so the change would be more invasive.
The FFMPEG_CODEC macro changes are a separate commit, and so easy to drop if necessary.
Builds with FFmpeg 0.11 on ArchLinux and FFmpeg 0.7 on Fedora 15.