Unset CXX, except on OSX.
Unset CXX, except on OSX.

joshua-cogliati-inl/raven,idaholab/raven,joshua-cogliati-inl/raven,joshua-cogliati-inl/raven,idaholab/raven,idaholab/raven,joshua-cogliati-inl/raven,idaholab/raven,idaholab/raven,joshua-cogliati-inl/raven,idaholab/raven,joshua-cogliati-inl/raven,idaholab/raven,joshua-cogliati-inl/raven