Adding FetchContent support

[Universumgames]

It would be nice if one could include OpenCL and add it the project as dependency via FetchContent.
That the CMake files are fixed to work with the separated directory structure of FetchContent and the libraries can be easily linked and directories added to the include paths.

[Universumgames]

I already tried something hacky in my fork [https://github.com/Universumgames/OpenCL-SDK/blob/fetchcontent/CMakeLists.txt](Custom CMakeLists.txt).
It works so far on my machine without any problems and I've tried it on a few other machines. The compilation works flawlessly both the normal build (as explained in the readme) and via FetchContent (or CPM)
I've created a gist for testing that [https://gist.github.com/Universumgames/5e18905c1885688dd7137047b78b458e](link to gist).
It took me a couple of tries to get it working reliable, but now it works well.