These headers are taken from Libav versions 0.8.9, 0.9 and 1.0.

These headers are licensed under the GNU Lesser General Public License version
2.1. For more information see the file COPYING.LGPLv2.1

While the function ABIs of FFmpeg/Libav tend to be stable between even major
versions, the class layouts can change considerably. This can lead to major
crashes if we build against the wrong headers. We include these headers to make
sure we have the same version every time we build, and to ease the pain of
acquiring the correct libraries for a build, especially for those distributions
who oppose distributing such packages.