Libav 11

Update to latest stable release, don't try to install nonexistent avserver files (avserver was dropped because it was "[unmaintained for years and largely broken](https://git.libav.org/?p=libav.git;a=blob;f=Changelog;hb=refs/tags/v11)").
We use ffmpeg to satisfy dependencies on these libraries, so I think we want to keep it as-is for now.