update util.hpp to fix OpenCL import on case sensitive mac FS

On a Mac with a case sensitive filesystem <OpenCL/OpenCL.h> does not exist but <OpenCL/opencl.h> does. I presume (!), but have no way to test, that on a Mac with case insensitive FS this change will make no difference.
update safe_call.hpp to fix OpenCL import on case sensitive mac FS

On a Mac with a case sensitive filesystem <OpenCL/OpenCL.h> does not exist but <OpenCL/opencl.h> does. I presume (!), but have no way to test, that on a Mac with case insensitive FS this change will make no difference.
Merge pull request #945 from malcolmreynolds:master