add GZDoom and Freedoom

Both packages should be build from the last Git versions:
https://github.com/freedoom/freedoom
https://github.com/coelckers/gzdoom

The package gzdoom-freedoom depends on freedoom-iwads, which is why I did include that package too.
GZDoom will be build with its own fmodex private libs.
If you want to keep those libs in a separate package, you could use the files in https://github.com/darealshinji/Debian/fmodex or https://github.com/darealshinji/Debian/fmodex-private to create such a package.