minor fix to handling of G4_NATIVE_VERSION

Geant4/G4config/CMakeLists.txt

@@ -26,7 +26,10 @@ if(DEFINED ENV{G4_NATIVE_VERSION})
   if(NOT Geant4_tag MATCHES "^4\\.")
     # G4_NATIVE_VERSION might not contain the initial '4.' that we need
     # see https://its.cern.ch/jira/browse/LHCBGAUSS-43
-    set(Geant4_tag 4.${Geant4_version})
+    set(Geant4_tag 4.${Geant4_tag})
+  endif()
+  if(NOT Geant4_tag MATCHES "^geant")
+    set(Geant4_tag geant${Geant4_tag})
   endif()
 else()
   set(Geant4_tag ${CMAKE_PROJECT_VERSION})