CMake: fix using Geant4 internal clhep

the target is now(?) prefixed with Geant4:: otherwise we get a linking error

cmake/DD4hepBuild.cmake

@@ -843,7 +843,11 @@ MACRO(DD4HEP_SETUP_GEANT4_TARGETS)
         PROPERTIES
         INTERFACE_INCLUDE_DIRECTORIES "${Geant4_INCLUDE_DIRS}"
       )
-      TARGET_LINK_LIBRARIES(CLHEP::CLHEP INTERFACE G4clhep)
+      if(TARGET Geant4::G4clhep)
+        TARGET_LINK_LIBRARIES(CLHEP::CLHEP INTERFACE Geant4::G4clhep)
+      else()
+        TARGET_LINK_LIBRARIES(CLHEP::CLHEP INTERFACE G4clhep)
+      endif()
 
     else()
       IF(NOT TARGET CLHEP::CLHEP)