Add support for OpenCL 1.1 C and C++ API

By default OpenCL is configured for NVIDIA CUDA. <PKG>/include and
<PKG>/lib64 are symlinks, which can me modified to point to
alternative implementation.

`opencl' and `opencl-cpp' are optional toolfiles and must be loaded
explicitly.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #477 from davidlt/opencl-support

Add support for OpenCL 1.1 C and C++ API