Add CUDA as a variant to OpenCV

[jsoller@…]

CUDA significantly increases OpenCV performance on MacBooks where NVIDIA cards and the CUDA library are installed.

In the graphics/opencv Portfile, the following lines could be added, allowing cuda to be selected as a variant. This requires the CUDA developer toolkit to be installed on the build machine.

variant cuda description {Enable CUDA support.} {

configure.args-delete -DWITH_CUDA=OFF configure.args-append -DWITH_CUDA=ON

}