COMP: Suppress CMake unused variable warnings
COMP: Suppress CMake unused variable warnings

For Slicer_DIR warning, Slicer_DIR variable is only added to TubeTK's
inner build configuration if TubeTK_USE_Slicer is on. TubeTK_Slicer_DIR is
factorized with it.

The same reasoning is applied for TubeTK_REQUIRED_QT_VERSION.

Closes issue #25.

Change-Id: Ib1de19790733eecff1ffe0883af7dce72d00655d

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