these variables seem useless in many cases. It was added
during intial superbuild support when CMAKE_PREFIX_PATH
was very less known.

Although there are cases, a force of variable is required,
those are exceptional ones which needs fix on either OTB side
or 3rd party side. Let's do that and avoid these hacks

Conflicts:
	Modules/Applications/AppClassification/include/otbTrainLibSVM.txx