ENH: Unset vars when switching between Slicer and stand-along builds
ENH: Unset vars when switching between Slicer and stand-along builds

Change-Id: Iee7b409c901f71f943773cc6fca756a4efd2cfb9

KitwareMedical/TubeTK,aylward/ITKTubeTK,KitwareMedical/ITKTubeTK,KitwareMedical/TubeTK,LucasGandel/TubeTK,matthieuheitz/TubeTK,sumedhasingla/TubeTK,cdeepakroy/TubeTK,KitwareMedical/ITKTubeTK,LucasGandel/TubeTK,thewtex/TubeTK,aylward/ITKTubeTK,sumedhasingla/TubeTK,matthieuheitz/TubeTK,aylward/ITKTubeTK,sumedhasingla/TubeTK,LucasGandel/TubeTK,aylward/ITKTubeTK,KitwareMedical/TubeTK,matthieuheitz/TubeTK,LucasGandel/TubeTK,KitwareMedical/ITKTubeTK,thewtex/TubeTK,thewtex/TubeTK,KitwareMedical/ITKTubeTK,cdeepakroy/TubeTK,thewtex/TubeTK,sumedhasingla/TubeTK,matthieuheitz/TubeTK,cdeepakroy/TubeTK,cdeepakroy/TubeTK,KitwareMedical/TubeTK