add libnice as a top-level subproject.

When relying on a system-wide libnice, we end up not building the nice elements, which means we can't use them, and by extension webrtcbin, in the uninstalled environment.

This also introduces a way to avoid checking the version of a given subproject, and makes use of it for libnice and pygobject, which only passed the version check by chance, as its current major version is 3.