[osg-users] OpenThreads Doxygen documentation

[Robert Osfield]

HI David,

Like Julian, I suspect that graphviz dot that doxgen uses to generate
the class diagrams isn't installed.

FYI, the OSG's github page has a link from the README.md to the
automatically generated doxygen docs:

    https://codedocs.xyz/openscenegraph/OpenSceneGraph/

Cheers,
Robert.