CMake, OpenCL: Added FindOpenCL detection script to cmake/ folder.

One no longer needs to copy FindOpenCL.cmake to some system path.
Should make the build process more automatic.
CMake: Replaced qt4_use_modules()-macro by target_link_libraries().

qt4_use_modules() is only available with CMake 2.8.11 or above, hence does
not work on many older systems.