[pkg-fgfs-crew] simgear/flightgear and openscenegraph fix

[Rebecca N. Palmer]

> I also included #767845 (grave, rebuild and tighten B-D on OpenSceneGraph).
It's simgear that needs the new openscenegraph, not flightgear, and 
given that it's already been binNMUd and we have no other reason to 
upload it, that may not be accepted.