Edward Z. Yang authored Mar 28, 2018 and Brad King committed Mar 29, 2018

This is useful if, for example, you want ccache to be used
for nvcc.  With the current behavior, cmake always picks up
/usr/local/cuda/bin/nvcc, even if there is a ccache nvcc
stub in the PATH.  Allowing for CUDA_NVCC_EXECUTABLE lets
us work around the problem.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>