Protect dart-gui-osg and check more rigorously for the presence of OSG (#898)
Protect dart-gui-osg and check more rigorously for the presence of OSG (#898)

Protect dart-gui-osg from being compiled when OSG is not present, and check more rigorously for the presence of OSG
dartsim/dart,dartsim/dart,dartsim/dart,dartsim/dart,dartsim/dart