[QGIS-Developer] OpenCL and macOS

[Tom Elwertowski]

For macOS, copying cl2.hpp from 
https://github.com/KhronosGroup/OpenCL-CLHPP/releases to 
/usr/local/include/CL works.

Setting USE_OPENCL=OFF also works to disable OpenCL.

One or the other is necessary or macOS will try the use the v1.2 C 
headers provided by Apple and pointed to by OpenCL_INCLUDE_DIR.

After lanuching QGIS, I see the Options|Acceleration panel with three 
choices:
Intel(R) Core(TM) i7-3615QM CPU @ 2.30GHz
HD Graphics 4000
GeForce GT 650M

To automate the mac build, my suggestion is to create a 
OpenCL_HPP_INCLUDE_DIR variable. If a mac user sets it, use OpenCL; if 
not, don't use OpenCL. For other platforms, OpenCL_HPP_INCLUDE_DIR is 
automatically set to OpenCL_INCLUDE_DIR.

Tom