ePub Media Overlay support requires libav v9 or later

The API for libav was changed from version 0.8.9 to version 9.
The 9.x APIs are the versions that are targetted by Cainteoir
Text-to-Speech.

Also, the libavresample library was first introduced in
version 9.

[1] http://upstream-tracker.org/versions/libav.html