Linking fix [fixes #15139]

Gotham build is currently broken on debian >=testing as well as other "bleeding edge" distros due to linking ffmpeg statically. To fix it we must make sure to link against all libs ffmpeg linked against.
Newer rtmp versions require nettle/hogweed and some other stuff that we don't check for.
Its a bit finicky to get that right, as we can't ask ffmpegs pkg-config files like we do on master since ffmpeg2

@t-nelson @jmarshall this really should go in before relase