COMP: FindDCMTK - Do not explicitly set DCMTK_DEFINITIONS

Following commit 04ee8cf1 fixing a packaging Debian bug,
the DCMTK_DEFINITIONS was set directly inside the FindDCMTK.cmake
module using "add_definitions".

When used outside of this project, setting automatically the definition
could have some unintended effects. For example, by globally adding
DCMTK_DEFINITIONS (and more specifically HAVE_CONFIG_H), it prevented from
compiling successfully program against the "tclap" library. Compile errors
like the following occurred:

// ------------------
In file included from <DIR>/SlicerExecutionModel/tclap/include/tclap/CmdLine.h:27:0,
                 from <DIR>/Slicer-build/Modules/CLI/CheckerBoardFilter/CheckerBoardFilterCLP.h:13,
                 from <SRC>/Modules/CLI/CheckerBoardFilter/CheckerBoardFilter.cxx:23:
<DIR>/SlicerExecutionModel/tclap/include/tclap/MultiSwitchArg.h:44:2: error: #error "Need a stringstream (sstream or strstream) to compile!"
 #error "Need a stringstream (sstream or strstream) to compile!"
  ^
<DIR>/SlicerExecutionModel/tclap/include/tclap/MultiSwitchArg.h:171:2: error: #error "Need a stringstream (sstream or strstream) to compile!"
 #error "Need a stringstream (sstream or strstream) to compile!"
  ^
In file included from <DIR>/SlicerExecutionModel/tclap/include/tclap/UnlabeledValueArg.h:30:0,
                 from <DIR>/SlicerExecutionModel/tclap/include/tclap/CmdLine.h:28,
                 from <DIR>/Slicer-build/Modules/CLI/CheckerBoardFilter/CheckerBoardFilterCLP.h:13,
                 from <SRC>/Modules/CLI/CheckerBoardFilter/CheckerBoardFilter.cxx:23:
<DIR>/SlicerExecutionModel/tclap/include/tclap/ValueArg.h:43:2: error: #error "Need a stringstream (sstream or strstream) to compile!"
 #error "Need a stringstream (sstream or strstream) to compile!"
  ^
<DIR>/SlicerExecutionModel/tclap/include/tclap/ValueArg.h:97:2: error: #error "Need a stringstream (sstream or strstream) to compile!"
 #error "Need a stringstream (sstream or strstream) to compile!"
  ^
// ------------------

Change-Id: I72357688765ce419541b4d893df870268da4b960
COMP: Associate ITKIODCMTK with DCMTK_DEFINITIONS

Following commits 04ee8cf and 361d2ea, DCMTK_DEFINITIONS was associated
with ITKIODCMTK target as a side effect of calling
"add_definitions(-D${DCMTK_DEFINITIONS})" in FindDCMTK.cmake.

Then, as explained in commit 0789e1d, the call to "add_definitions" has
been removed to prevent compile side effect in project building against ITK
and using the FindDCMTK.cmake provided by ITK.

This commit accounts for that change by explicitly associating the
DCMTK_DEFINITIONS with ITKIODCMTK target.

Change-Id: Ia21dd65eaa0aadaf2f2eafb034aea48b42d71a8c
Merge topic 'fix-building-against-system-dcmtk'

efbca71 COMP: Associate ITKIODCMTK with DCMTK_DEFINITIONS
b68df3a COMP: FindDCMTK - Do not explicitly set DCMTK_DEFINITIONS