CuPy requires --cuda-compute-capabilities flag to install on some systems

[stevecdavis]

Installing CuPy on a system that does not have cuda_cc_cmake variable set throws an error requiring the flag --cuda-compute-capabilities flag to be set.

It'd be beneficial if there are no cuda compute capabilities specified it'd throw an error and not crash hard.
It might also be helpful if the error gave a little direction i.e. -
"Look here to confirm your compute capabilities - https://en.wikipedia.org/wiki/CUDA#GPUs_supported"