COMP: Ensure DCMTK_DIR is set in SlicerConfig.cmake

Introduced in r20634, the SlicerConfig variable DCMTK_DIR has never been
set. This commit fix this by updating SlicerGenerateSlicerConfig.cmake
and setting variable DCMTK_DIR_CONFIG.

Extensions should now be able to use variables DCMTK_LIBRARIES
and DCMTK_INCLUDE_DIRS directly after doing: find_package(Slicer)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21854 3bd1e089-480b-0410-8dfb-8563597acbee