e.  You must also opt into OpenGL and OpenThreads (and Producer
if needed) as these modules won't do it for you.  This is to allow you
control over your own system piece by piece in case you need to opt
out of certain components or change the Find behavior for a particular
module (perhaps because the default FindOpenGL.cmake module doesn't
work with your system as an example).  If you want to use a more
convenient module that includes everything, use the
FindOpenSceneGraph.cmake instead of the Findosg*.cmak