FFmpeg in-tree build for mac.

third_party/ffmpeg/README.chromium

 This file describes how to produce the FFmpeg include directory, and how to
 create the ffmpeg.gyp file and related configurations.
 
 -- FFmpeg headers in the 'include' directory.
 
 The include directory contains FFmpeg's public header files from the output of
 a "make install" command.  The header files are from Chromium's copy of FFmpeg.
 
 Steps to reproduce:
   1) If on Windows, refer to our MinGW/MSYS environment setup:
@@ skipping 112 matching lines @@
      on config.h files for other platforms (in particular, for x64) will
      likely result in unecessarily slow code, or compile failures.
 
   4) Next, capture all the output from a build of libavcodec.so and
      libavformat.so.  We will use the build log as a reference for making
      the ffmpeg.gyp file.
 
        make libavcodec/libavcodec.so libavformat/libavformat.so \
           > ffmpeg_build_log 2> ffmpeg_build_err
 
+     For Mac, replace the ".so" in the files above with ".dylib".
+
   5) Check ffmpeg_build_err to see if there are any significant
      anomalies.  FFmpeg source generates a lot of compiler warnings; it
      is safe to ignore those.
 
   6) Examine all non-gcc commands to see if we're missing anything
      interesting:
 
        grep -v '^gcc' ffmpeg_build_log
 
      There should be yasm commands for assembling two yasm files, but nothing
@@ skipping 52 matching lines @@
        grep -v ' -shared ' |
        sed -e "s|.* -o .* \(.*\)$|'source/patched-ffmpeg-mt/\1',|" |
        sort
 
  11) Attempt to build. :)
 
 *12) Update the the sources! clause to exclude files that should only be built
      for Chromium.  For this, you basically need to do the steps above once
      with the configure options for Chrome, then once with the options for
      Chromium and diff the list of .c and .asm source files.