You must have libavcodec installed to use this library; it was already on my machine.
libavcodec58 - FFmpeg library with de/encoders for audio/video codecs - runtime files
{cnuernber/avclj {:git/url "https://github.com/cnuernber/avclj"
:sha "HEAD"}}
FFmpeg provides a function to get the license of the library; it returns GPLv2 on my system. This library is thus transitively licensed under GPLv2.