Updated March 28, 2014
The clhep environment is set up automatically with the other packages using the JLab common environment scripts, for example:
source /site/12gev_phys/softenv.csh 2.2
The supported geant4 versions can be found here.
The JLab common environment provides utilities to compile clhep applications using scons.
The SConstruct file should include the lines:
from init_env import init_environment env = init_environment("clhep")
This will load the clhep environment in scons.
A step-by-step guide on how to install clhep can be found here.
If you want a clhep feature not included in the standard release I'll be happy to help implement it.