`libmp4v2-dev` no longer exists in Ubuntu / Mint

[vassilisw]

On Mint 20 (i.e. Ubuntu 20), seems that libmp4v2-dev is deprecated...

Please remove mp4v2, it's dead upstream and better alternatives exist. The last remaining use case in the archive has been fixed to no longer use it.

So, we have to download it separately from somewhere like here.