Modifying gazebo_ros_pkgs to build a custom version of Gazebo

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Hei guys, I am not sure where to modify gazebo_ros_pkgs in order to have my modified version of Gazebo built. As far as I got I see that there is a file gazebo_ros_pkgs/gazebo_dev/cmake/gazebo_dev-extras.cmake that uses some environment variables in order to add the source files.

# The following lines will tell catkin to add the Gazebo directories and libraries to the
# respective catkin_* cmake variables.
set(gazebo_dev_INCLUDE_DIRS ${GAZEBO_INCLUDE_DIRS})
set(gazebo_dev_LIBRARY_DIRS ${GAZEBO_LIBRARY_DIRS})
set(gazebo_dev_LIBRARIES ${GAZEBO_LIBRARIES})

Is that it? If so do I just need to set those up, dowload gazebo_ros_pkgs and then build the workspace?

Tnx