Hard-coded CUDA path

[sbliven]

The path to CUDA is currently hard-coded to /usr/local/cuda-5.5/include:

https://github.com/C-CINA/focus/blob/495463215f56d86203f0e2f9e54a7af9b55e9e4c/CMakeLists.txt#L84

This should be made configurable with an environmental variable, e.g. $CUDA_INCLUDE_DIR