Stop using custom FindOpenCL.cmake, per TODO
It was originally taken from cmake 3.1.0 and modified to produce
all-uppercase variable names (with OPENCL_ instead of OpenCL_),
so the surrounding cmake code is now changed to expect exact-case names.
Another (undocumented) modification was that it took into account
CUDA_HOME env.var. when searching for OpenCL, while standard cmake
only looks at seemingly less useful CUDA_PATH. This useful trick is
preserved by temporarily appending $ENV{CUDA_HOME} to CMAKE_PREFIX_PATH.
Change-Id: I5ca60f88e657b064fb6f98b268c2c48e10f00986