src/scripts/cuda.{ac,mk.in}: added a --with-cuda-level option

For specifing the CUDA compute capability to use when compiling.
.../hybridAPI/Makefile: add naive fixed CUDA-level `nvcc` flags

This is better than nvcc's default, which uses compute capability 1.0,
but may not be suitable for all applications.  Also added a comment
pointing to more information on less-naive ways to specify target CUDA
capability.