cuda.ac: update docs to reflect > 1 `--with` arg
cuda.{ac,mk.in}: added a --with-cuda-level option

For specifying the CUDA compute capability to use when compiling.
Makefile.in: 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 in all circumstances.  Also added a comment
pointing to more information on less-naive ways to specify target CUDA
capability.