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 Section 5.2. (Appendix - Makefiles and Environment Variables) of the Geant4 User's Guide for Application Developers , you can find a list of all the environment variables. In Section 5.3 it is also explained how to integrate external libraries which may or may not use the Geant4 kernel libraries, using the GNUmake build system of Geant4.