pycuda uses unavailable compute capabilities on older versions of CUDA with new hardware

pycuda defaults to asking nvcc to use the maximum compute capability available on the GPU. This fails if the version of CUDA doesn't support the compute capability. For instance, if you're trying to use a GTX 1080 on CUDA 7.5 you get error messages like:

`ExecError: error invoking 'nvcc --preprocess -arch sm_61 -Ifile.cu --compiler-options -P': [Errno 2] No such file or directory`

The solution seems to be to use the highest compute capability available in CUDA that's supported by the card, but I'm not sure the best way to do that.