BUG: USE_SLICER (not BUILD_SLICER_EXT) flags if Slicer provides ITK, etc.
Merge pull request #187 from aylward/ITKv4WhenTubeTK_USE_SLICER

BUG: USE_SLICER (not BUILD_SLICER_EXT) flags if Slicer provides ITK, etc...