opencv: Switch to an option to disable OpenCL

OpenCV 2.4.5 enables OpenCL by default (on most platforms), so switch to
having an option to disable the OpenCL module.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>
opencv: Automatically disable OpenCL for pre-Lion

In particular, OpenCL 1.1 is required by OpenCV 2.4.5+, which is
unavailable in any pre-Lion version of OS X.

Closes #206.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>