You can include "gazebo/gazebo_config.h" which contains the following #defines:

#define GAZEBO_MAJOR_VERSION 1

#define GAZEBO_MINOR_VERSION 4

#define GAZEBO_PATCH_VERSION 0

#define GAZEBO_VERSION "1.4"

#define GAZEBO_VERSION_FULL "1.4.0"

The numbers will obviously be different.