Wordy title, sorry.

I'm trying to adapt the code from this tutorial: http://gazebosim.org/tutorials?cat=guided_i&tut=guided_i6

I want it, instead to subscribe to a topic containing a std_msg, to subscribe to a topic containing a custom message that I defined in a separate ROS package.

I can (probably) get the c++ code to handle this correctly by looking at tutorials for subscribing to custom messages online. However, to compile this, I would need to include my ROS package containing the custom message as a dependency somehow and I'm a total CMake noob. So, to sum up, what do I need in my CMakeLists file to support what I'm trying to do, here?