The use of nvcc to compile cuda kernels is distinct from the use of
cuda sdk libraries and linking against those libraries. We have
previously not bothered to distinguish these two cases because all
the filters that used cuda kernels also used the sdk. In the following
changes, I'm going to remove the sdk dependency from those filters,
but we need a way to ensure that nvcc is present and functioning, and
also a way to explicitly disable its use so that the filters are not
built.
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>