I just did some googling but I couldn't find anything conclusive about which one is right... I think they both work? I searched through the my ROS workspace and lots of code uses the singular ``_DIR`` include:

```
./ros_control/controller_manager_tests/CMakeLists.txt:34:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./ros_control/controller_manager/CMakeLists.txt:21:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./gazebo_tutorials/CMakeLists.txt:18:include_directories(${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS} ${GAZEBO_INCLUDE_DIRS})
./control_toolbox/CMakeLists.txt:40:    ${Boost_INCLUDE_DIR}
./ros_controllers/joint_state_controller/CMakeLists.txt:19:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./ros_controllers/position_controllers/CMakeLists.txt:20:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./ros_controllers/effort_controllers/CMakeLists.txt:21:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./ros_controllers/imu_sensor_controller/CMakeLists.txt:18:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./ros_controllers/force_torque_sensor_controller/CMakeLists.txt:18:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./ros_controllers/velocity_controllers/CMakeLists.txt:19:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./realtime_tools/CMakeLists.txt:16:  include_directories(include ${Boost_INCLUDE_DIR} ${catkin_INCLUDE_DIRS})
./gazebo_ros_pkgs/gazebo_ros_control/CMakeLists.txt:29:  ${Boost_INCLUDE_DIR}
```