Adding missing OpenCL support

Recent OCCT670 features (shaders, raytracing) rely on OpenCL and are available if and only if HAVE_OPENCL is defined.

This branch adds OpenCL support. It properly compiles on OSX, don't know if it works though, I need a few DRAW commands to check it out.
@thatcadguy This file has been copied from an external repository; I suggest that we leave it as is, and modify it only if needed and then send our fix back to findopencl's author.