configure, external ffmpeg: Drop check for libavresample.
configure: Add --enable-libav-compat option.
Add a fake .pc file for libswresample -> libavresample
DllSwResample: Clean up libavresample support that is broken anyway.
Introduce a libav-hacks library to wrap a compatibility layer with libav.

This copies/pastes some code from FFmpeg but at least allows us to support the fork.
DllAvFormat: map av_stream_get_r_frame_rate when available.
Use av_stream_get_r_frame_rate instead of accessing avstream->r_frame_rate when
available.

From avformat.h:
     * Code outside avformat should access this field using:
     * av_stream_get/set_r_frame_rate(stream)
ActiveAE: Replace deprecated CODEC_ID by AV_CODEC_ID.

The former does not build with FFmpeg 2.0 and I somehow missed this reading the code.
Maybe I should remove the compatibility layer with the old names in the internal FFmpeg to prevent this to happen again.
DllAvUtil: Drop libav hacks and move them to libav_hacks.h.
Move libav check for libavfilter being avframe based to libav_hacks.h
Remove avfilter_graph_parse hacks for libav and move it to libav hacks
DVDVideoCodecFFmpeg: Drop usage of LIBAVFILTER_FROM_* and use LIBAVFILTER_AVFRAME_BASED instead.
DllAvFilter.h: Drop checks for libav and factorize some #if / #else. Those are now done in libav hacks.
DVDOverlayCodecFFmpeg.cpp: Remove pointless #ifery since we require a recent enough FFmpeg anyway.
DVDOverlayCodecText: Remove libav compat #ifery and move it to libav_hacks.h
DVDFactoryCodec: Remove pointless libav #ifery and improve the libav_hacks.h
hackery so that it actually compiles with libav.
DllAvCodec: Remove now unused LIBAVCODEC_FROM_* macros.
Fix swr_get_delay computation from libavresample API.
libav hacks: define AVFORMAT_HAS_STREAM_R_FRAMERATE to give it a dummy accessor.
Merge pull request #3036 from aballier/libavhacks

Add a libav compatibility layer with --enable-libav-compat