As part of the Geant4 kernel libraries installation, it is also possible to put the entire set of header files in a single place, which is determined by the environment variable G4INCLUDE specifying the directory path. Therefore, it's rather straightforward to integrate Geant4 into a generic external framework, by simply knowing the path where header files are located in the system (G4INCLUDE) and where installed libraries are placed (G4LIB).

In Appendix - Makefiles and Environment Variables of the Geant4 User's Guide for Application Developers , you can find a list of all the environment variables, together with a section explaining how to integrate external libraries which may or may not use Geant4 kernel libraries, in the GNUmake mechanism of Geant4.