I'm just following the instructions, and have installed libav successfully. You have to make sure you have added the sources in your sources.list file, like so:
nano /etc/apt/sources.list
Uncomment all lines in this file, then save and try again installing...
sudo apt-get build-dep libav
------------------
Unfortunately I'm not having luck compiling it with the new OPI Zero... I'm gonna try on my side to find a solution and post a new thread specific to the OPI Zeron install...