And if you have, you might need to point cmake to the installation using CMAKE_PREFIX_PATH. For example, I have the following cmake options (but your Qt might be somewhere else):
In addition to @allison’s comment, it looks like you’re trying to build the example with the old “Geant4GNUmake” system. Did you source the geant4make.sh script before trying to build?
In either case, it’s strongly recommended to use the CMake build system unless you need to maintain an existing GNUmake system. See the Application Developer Guide for how to use this.