[travis] simbody make doxygen only if DOXY.
[travis] simbody make doxygen only if DOXY.
opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core