added -lpthread to the link line for Linux builds.  Seems Red Hat 7.3 turns up undefined symbols without it.
added -lpthread to the link line for Linux builds.  Seems Red Hat 7.3
turns up undefined symbols without it.


git-svn-id: 23b6355f2bb369032457de4eb5f713ee134f73a8@862 16af8721-9629-0410-8352-f15c8da7e697

joevandyk/osg,joevandyk/osg,joevandyk/osg,joevandyk/osg,joevandyk/osg