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
Merge topic 'DeleteCMakeWarning'

09945eb COMP: Suppress CMake unused variable warnings