Fix load of geant4 libraries

Load of GEANT4 libraries via script gconfig/g4libs.C is outdated. It is not a problem since we don't support GEANT4 for now, but when trying to re-enable the support for GEANT4, we will need to fix the script g4libs.C. As a starting point, good inspiration can be found in alisw: https://github.com/alisw/geant4_vmc/blob/master/examples/macro/g4libs.C