Do use ninja for Geant4 builds!

CMakeLists.txt

 CMAKE_MINIMUM_REQUIRED(VERSION 2.8.5)
 
-# NO_NINJA : Geant4 build fails with Ninja
+# USENINJA : Geant4 build should not fail with Ninja
 
 set(GEANT4_SRCS_REPOSITORY https://gitlab.cern.ch/lhcb/Geant4-srcs.git CACHE STRING "Repository for Geant4 sources")