When using `pkg-config`, we're [working around](https://github.com/gerkey/ros1_external_use/blob/master/ros1_comms/Makefile#L9-L14) a pre-existing issue, specifically ros/catkin#694. It was fixed for internal-to-ROS use in ros/ros#87, but the [problem persists for external-to-ROS use](https://github.com/ros/catkin/issues/694#issuecomment-128291617). We should actually fix that problem.