COMP: Fix build error in External_DCMTK.cmake

Reported-by: Erich Bremer <erich.bremer@stonybrook.edu>
Thanks: Steve Pieper <pieper@bwh.harvard.edu>

This commit fixes a regression introduced in r24483 (BUG: fix support
for DCMTK DICOM dictionaries) where a line continuation character
is used. It turns out that this character caused parsing issue with older
version of CMake (< v3.0.0)

//-------------------------------
CMake Error at SuperBuild/External_DCMTK.cmake:99 (set):
Syntax error in cmake code at

 /home/erich/MyProjects/trunk/SuperBuild/External_DCMTK.cmake:100

when parsing string

 DCMDICTHOME=<APPLAUNCHER_DIR>/${Slicer_SHARE_DIR}/dicom.dic<PATHSEP>\

<APPLAUNCHER_DIR>/${Slicer_SHARE_DIR}/private.dic

syntax error, unexpected cal_SYMBOL, expecting $end (89)
Call Stack (most recent call first):
CMake/ExternalProjectDependency.cmake:593 (include)
SuperBuild/External_ITKv4.cmake:14 (ExternalProject_Include_Dependencies)
CMake/ExternalProjectDependency.cmake:593 (include)
CMake/ExternalProjectDependency.cmake:639 (ExternalProject_Include_Dependencies)
SuperBuild.cmake:323 (ExternalProject_Include_Dependencies)
CMakeLists.txt:670 (include)
//-------------------------------

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24490 3bd1e089-480b-0410-8dfb-8563597acbee