STYLE: Add Base subdirectory conditionally
STYLE: Add Base subdirectory conditionally

This commit makes the add_subdirectory call from Base/CMakeLists.txt
conditional. This enables ITKTubeTK to request build only for the
Base libraries it needs, so the integration of TubeTK in ITK is easier.

Co-authored-by: Jean-Christophe Fillion-Robin <859d3fd8ca46de62a08fca56c64958e301f5352c@kitware.com>

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