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)").
I assume that the reason we're manually installing the binaries and man pages instead of installing everything is to avoid conflicts with ffmpeg's libraries and header files, is that correct?

Would a better solution be to install libav in its entirety (with ```make install```) but add ffmpeg as a conflict so that only one is linked at a time?