(io): make realsense support optional
Description
Realsense camera are great, but everybody do not own one. On Linux, compiling supporting for realsense force people to install a PPA repository. Mainly because of this, I think it should be an optional feature.
Proposal
Add a CMake option to enable/disable support (disabled by default), similarly to openvslam. This option will be used to enable/disable module_io_realsense and ExRealsense. It should also be used in ExRGBDImageMasking and SightCalibrator.
Outcomes
Accessibility improved for newcomers.