Changes to FindOpenCL.cmake module

Hey Kyle,

I made a few changes to the FindOpenCL module that make this work better on my machine. I think this may be generally useful. The strategy is to search in the following locations
- A user specified OpenCL_ROOT_DIR (e.g. cmake -DOpenCL_ROOT_DIR=/opt/AMDAPP source_dir)
- If none is given the $CUDA_TOOLKIT_ROOT_DIR is searched
- If that can't be found search in $AMDAPPSDKROOT
In addition the normal system path is search (e.g. /usr/lib64 etc.)

Cheers,
Dominic