This extends FindOpenShadingLanguage.cmake to also look for the location of
stdosl.h and adds the path to the invocation of oslc to deal with the headers
being in different locations a little better.
Differential Revision: https://developer.blender.org/D6865
Reviewers: brecht
Reduce the number of possible locations used to find libraries,
to simplify troubleshooting.
Only keep '*_ROOT_DIR' and the path used by 'install_deps.sh'.
