> add_dependencies(robot_sim ${PROJECT_NAME})

Hmm, that shouldn't fix anything. In the following lines from the
CMakeLists.txt, the call to target_link_libraries() causes the
executable to depend on the library:

rosbuild_add_library(${PROJECT_NAME} ${SIM_LIB})
rosbuild_add_executable(robot_sim src/robot_sim.cpp)
target_link_libraries(robot_sim ${PROJECT_NAME})

There shouldn't be any missing dependencies. Can you post the build
errors that you see on the first 'make' invocation?

Ah, I just learned something - feel like a beginner. I've been doing my target linking prefixing a '-l' to the library name, which works, but doesn't let cmake use what it knows about the internally built library. In this case, it needs the add_dependencies.