Fix bug in FindOpenCL.cmake. We shouldn't be overwriting the variable
set by find_package_handle_standard_args().
Add helpful output showing where detected include directories and libraries
in FindOpenCL.cmake
Fix building under linux if we only have the OpenCL header files
available but don't have the OpenCL library installed.

Note on OSX we enforce that the library is present because it is
required for our implementation.