message: Avoid CMake policy CMP0072 warning
  Prefer libGL over libOpenGl for better compatibility with SWR.
diff: ## limitations under the License. ##
  ## ======================================================================== ##
  +# avoid CMake policy CMP0072 warning:
  +# prefer libGL over libOpenGl for better compatibility with SWR
  +set(OpenGL_GL_PREFERENCE "LEGACY")
  find_package(OpenGL REQUIRED)
  ospray_disable_compiler_warnings()
  