CMakeLists: properly handle the GEOMODEL_BUILD_EXAMPLES_W_GEANT4 option

The GEOMODEL_BUILD_EXAMPLES_W_GEANT4 option was working only when also the GEOMODEL_BUILD_EXAMPLES option was activated. Now:

• if both options are active, only one of the 2 is taken into consideration to avoid double inclusion of folders
• if only the GEOMODEL_BUILD_EXAMPLES_W_GEANT4 is set, the examples are built

This also allows the CI to test properly the Geant4 examples which were not built and tested before