conditional addition of VTKm_OPENLGL_INCLUDE_DIRS
On a system that has no opengl or mesa and with rendering enabled (such as docker container), vtkm fails to configure when adding target_include_directories for the rendering library. I believe this happens because cmake determines that the variable VTKm_OPENGL_INCLUDE_DIRECTORIES is dependent on OPENGL_INCLUDE_DIRS( which is NOTFOUND). This causes cmake to raise an error even when VTKm_OPENGL_INCLUDE_DIRECTORIES is set to "".
This fixes the issue.