Allow user to turn off DDG4 requirement.

For applications not requiring Geant4 but that do require the geometry we should be able to build this without the Geant4 dependency. Default is same as before (DDG4 required) but user may turn off the requirement with the -DUSE_DDG4=OFF option.

CMakeLists.txt

@@ -31,8 +31,20 @@ endif()
 # Export compile commands as json for run-clang-tidy
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
+# Set required DD4hep components based on user input
+set(USE_DDG4 "TRUE" CACHE BOOL "Require DD4hep DDG4 component")
+set(DD4hep_required_components DDCore DDRec)
+if(${USE_DDG4})
+  list(APPEND DD4hep_required_components DDG4)
+endif()
+
+# The items passed to target_link_libraries below need DD4hep:: prepended
+set(DD4hep_required_libraries ${DD4hep_required_components})
+list(TRANSFORM DD4hep_required_libraries PREPEND DD4hep::)
+
+
 # Dependencies
-find_package(DD4hep REQUIRED COMPONENTS DDCore DDG4)
+find_package(DD4hep REQUIRED COMPONENTS ${DD4hep_required_components})
 find_package(ActsDD4hep)
 if(ActsDD4hep_FOUND)
   add_compile_definitions(USE_ACTSDD4HEP)
@@ -54,7 +66,7 @@ dd4hep_add_plugin(${a_lib_name}
   USES ${ActsDD4hep} ROOT::Core ROOT::Gdml
   )
 target_link_libraries(${a_lib_name}
-  PUBLIC DD4hep::DDCore DD4hep::DDRec DD4hep::DDG4
+  PUBLIC ${DD4hep_required_libraries}
   )
 
 #-----------------------------------------------------------------------------------