Minor fixes

This is just to fix a couple of minor annonyances:

• Geant4 can be build with Ninja now, so no reason to disable it
• we can build without setting $CMTCONFIG so relying on it is a bad idea
• somebody had the clever idea of adding git to the LCG stack, breaking the compilation in dev3/4, so I had to filter it away

/cc @kreps @adavis @gcorti