I am not an expert in using cmake (my experience is with menuconfig) and my exposure to cmake is limited to opencv.
As I need to choose between different package config files (gtk+-2.0 or gtk+-3.0) depending on which system is chosen I decided to copy the method used for distinguishing between QT4 and QT5 (HAVE_QT and HAVE_QT5). This provides a consistency in approach for configuring QT or GTK (this can been seen in cmake/OpenCVFindLibsGUI.cmake).