[cmake] add CUDA support to cmake
[cmake] add CUDA support to cmake

1. Update top level CMake to add support for CUDA.
2. Exposed a cmake flag to enable CUDA.
3. Currently requiring a minimum of CUDA 9.0 (may change later).
4. Because cmake minimum version is 3.2, having to switch between
add_executable and cuda_add_executable based on flag. cmake 3.9+ has
neater support.

isovic/racon,isovic/consise,isovic/consise,isovic/racon