dan.j.williams, the Linux distribution that is on the SoC board does not have OpenGL installed. Also, the ARM cross-compiler tool chain that we ship does not have OpenGL support. That's why gl.h is not found. Of course, you probably don't care about OpenGL at all, only C++ OpenCL interface.
The best thing I can think of is to modify cl.hpp to not include gl.h (cl.hpp is created by the Khronos group and we do not modify it). Give it a quick try. I will try to do the same to see if I can get you through quickly.
We did use cl.hpp on non-SoC platform and it works. So that's the only issue you should see with the C++ interface.