add libnice as a top-level subproject.

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.