COMP: Tweak installation of DCMTK libraries for MacOSX packaging

Following commit r21914, DCMTK libraries are build as shared
library. Commit r21944 ensured that they were installed. Since on MacOSX
the fixup process takes care of copying the library associated with
external projects, this commit exclude the systematic installation
of DCMTK libraries and teach the fixup script where these libraries
are located.

See issue #3040

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21947 3bd1e089-480b-0410-8dfb-8563597acbee