COMP: Ensure FindDCMTK.cmake provided by ITK is used (build tree case)

When building project against ITK, if the CMAKE_MODULE_PATH is not
updated to include the location of the FindDCMTK.cmake module provided
by ITK, the default one provided by CMake will be used and the
configuration will fail with an error like the one reported below.

Note that this commit only fix the "exported build tree" case, similar
patch should be done for the "exported install tree" case (including
the addition of an install rule for the FindDCMTK.cmake module provided
by ITK)

Finally, it is also agreed that the module provided within CMake should
be fixed. For more detailed, see commit d9034833.

Configuration error:

CMake Error at /home/jchris/Support/cmake-2.8.11.2/Modules/FindPackageHandleStandardArgs.cmake:108 (message):
  Could NOT find DCMTK (missing: DCMTK_ofstd_INCLUDE_DIR
  DCMTK_dcmdata_INCLUDE_DIR DCMTK_dcmimgle_INCLUDE_DIR)
Call Stack (most recent call first):
  /home/jchris/Support/cmake-2.8.11.2/Modules/FindPackageHandleStandardArgs.cmake:315 (_FPHSA_FAILURE_MESSAGE)
  /home/jchris/Support/cmake-2.8.11.2/Modules/FindDCMTK.cmake:138 (find_package_handle_standard_args)
  /home/jchris/Projects/Slicer-SuperBuild-Debug-Qt485-new-buildsystem/ITKv4-build/lib/cmake/ITK-4.5/Modules/ITKDCMTK.cmake:9 (find_package)
  /home/jchris/Projects/Slicer-SuperBuild-Debug-Qt485-new-buildsystem/ITKv4/CMake/ITKModuleAPI.cmake:48 (include)
  /home/jchris/Projects/Slicer-SuperBuild-Debug-Qt485-new-buildsystem/ITKv4/CMake/ITKModuleAPI.cmake:25 (itk_module_load)
  /home/jchris/Projects/Slicer-SuperBuild-Debug-Qt485-new-buildsystem/ITKv4/CMake/ITKModuleAPI.cmake:67 (_itk_module_config_recurse)
  /home/jchris/Projects/Slicer-SuperBuild-Debug-Qt485-new-buildsystem/ITKv4-build/ITKConfig.cmake:81 (itk_module_config)
  /home/jchris/Support/cmake-2.8.11.2/Modules/FindITK.cmake:48 (find_package)

Change-Id: I18240028092bd3b7879db18e6d05aa0c1669f5b6
Merge topic 'fix-building-against-system-dcmtk'

d6ca29c COMP: Ensure FindDCMTK.cmake provided by ITK is used (build tree case)