Build some binaries only when SDK is installed

A fix to https://github.com/ros-drivers/leap_motion/issues/7.

When SDK isn't available, catkin skips to build a couple of binaries that are introduced in https://github.com/ros-drivers/leap_motion/pull/3. That said, DEB won't included those binaries since AFAIK I don't think Leap SDK is available on ROS buildfarm (and won't be soon).

Fyi @YuOhara