cuda.{mk.in,ac},configure,Makefile.in: more CUDA `configure` options, docs

  * Moved `configure`-time CUDA configuration values to `cuda.mk.in`.
  * Added --with-cuda-sdk=PATH option => CUDA_SDK_DIR in cuda.mk
  * Added additional documentation to cuda.ac
  * Added HAVE_CUDA and HAVE_CUDA_SDK vars to cuda.mk