COMP: Fix unused CMake variable occurring when configuring DCMTK on windows

Warning reported on the dashboard:

12> Manually-specified variables were not used by the project:
12>    DCMTK_FORCE_FPIC_ON_UNIX
12>    DCMTK_WITH_WRAP

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21992 3bd1e089-480b-0410-8dfb-8563597acbee