DDG4: enable building with geant4 internal CLHEP

CMakeLists.txt

@@ -126,6 +126,11 @@ add_subdirectory(DDDetectors)
 add_subdirectory(DDCond)
 add_subdirectory(DDAlign)
 if ( DD4HEP_USE_GEANT4 )
+  if(Geant4_builtin_clhep_FOUND)
+    set(CLHEP "")
+  else()
+    set(CLHEP CLHEP)
+  endif()
   add_subdirectory(DDG4)
 endif()
 # Note the order: DDEve partially depends on variables from DDG4!

DDG4/CMakeLists.txt

@@ -12,7 +12,7 @@ dd4hep_package(    DDG4
   USES            [GEANT4 REQUIRED COMPONENTS gdml ui_all vis_all]
                   [ROOT   REQUIRED COMPONENTS Geom GenVector RIO]
                    BOOST DDCore
-  OPTIONAL         CLHEP [XercesC QUIET]
+  OPTIONAL         ${CLHEP} [XercesC QUIET]
   INCLUDE_DIRS     include
   INSTALL_INCLUDES include/DDG4 )
 MESSAGE(STATUS "CLHEP_LIBRARIES ${CLHEP_LIBRARIES} ${CLHEP_LIBRARY}")