DllAvCodec.h: remove mention of libavcore that is long gone.
configure: Do not check for libavcore files since it is long gone and xbmc requires ffmpeg 1.0
Remove the checks for AVPacket definitions in libavformat.

It has been moved to libavcodec in 2009, libavcodec version 52, and xbmc requires ffmpeg 1.0, libavcodec version 54, so we can assume AVPacket is always defined in libavcodec.
Drop checks for libav*/opt.h.

It has been moved to libavutil/opt.h in 2010, libavutil versin 50, and xbmc requires ffmpeg 1.0, libavutil version 51.
Remove rgb2rgb.h checks.

This header is unused in xbmc code.
Remove the check for libavutil/mathematics.h and assume it is always present.

This header has existed since years and xbmc will not build with such an old ffmpeg version that does not have it.
Remove the check for libavutil/mem.h and assume it is always present.

This header has existed since 2007 and xbmc will not build with such an old ffmpeg version that does not have it.
Remove the check for libavutil/samplefmt.h and assume it is always present.

This header has existed since libavcore got merged back into libavutil and xbmc will not build with such an old ffmpeg version that does not have it.
Drop support for old ffmpeg header layout.

xbmc requires a ffmpeg version that has the new layout.
Drop PIX_FMT_VDPAU_MPEG4 checks and assume it is always present. It is the case for all ffmpeg versions xbmc can build against.
configure: remove a no longer valid comment
Restore code parts from [573b4bc18] to improve libav compatibility.
configure: Drop checks for sample_aspect_ratio. They are useless because they are done via #ifery now.
Merge pull request #2760 from aballier/ffmpeg_cleanups

External FFmpeg cleaning