If the user has CUDA installed, we run a little program to see if
 a. the CUDA install works - file compiles
 b. that if there is a CUDA device installed we:
    i. set the correct CC in sac2crc, or
   ii. set a default '-arch=sm_35' seems as good as any choice
 c. The CUDA install does not work, we *do not* activate CUDA

Furthermore we add a test to make sure that the CUDA backend is
generating kernels.