CL/cl2.hpp, which the OpenCL support is based upon, is not always
included with OpenCL on some platforms, e.g. Mac, or not readily
available as a package. This work adds a CMake module specifically for
finding cl2.hpp, as installed by OpenCL-CLHPP project.
If not found, but standard OpenCL lib and headers are, the vendored
cl2.hpp in external/opencl-clhpp is used, as it needs no compilation.
- Only the cl2.hpp, license and README are vendored from OpenCL-CLHPP.
- Fix up referenced includes in other CMake targets, to ensure the
includes for OpenCL are specifically added (previously, they were
sometimes found in existing include directories of other dependencies).
- Fixup for standard FindOpenCL module not assinging proper framework
headers directory for Mac.
