OpenCL: Do not assume that OpenCL framework is always used in Apple
On Apple, we can use the OpenCL loader from https://github.com/KhronosGroup/OpenCL-ICD-Loader and the headers from https://github.com/KhronosGroup/OpenCL-Headers.
On Apple, we can use the OpenCL loader from https://github.com/KhronosGroup/OpenCL-ICD-Loader and the headers from https://github.com/KhronosGroup/OpenCL-Headers.