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
actually, that one was for GCC v4.x and up too, so this patch only extends on the `unistd` patch

keeping the `unistd` around though, for backward compatibility (i.e. for easyconfigs still using only the `unistd` one)