fix & clean up GATE v6.x easyconfigs, fix build problem with CLHEP 2.1.1.0

requires https://github.com/hpcugent/easybuild-easyblocks/pull/517 (Geant4 v9.4) and https://github.com/hpcugent/easybuild-easyblocks/pull/518 (GATE)

this fixes broken CHLEP build on SL6, cfr. #641
the makefile has no default `PREFIX`?