Ensure DCMTK libraries can be found on system without setting DCMTK_DIR

This commit ensure that DCMTK_DIR is set to a reasonable default value
so that DCMTK libraries can be found on a standard Unix distribution.

It also overwrite the value of DCMTK_DIR after this one has been
set by a successful discovery of DCMTK by the FindDCMTK.cmake module
bundler within CMake (0167cea)

See #382
Merge branch '382-fix-find-dcmtk-issue' into pu

* 382-fix-find-dcmtk-issue:
  Ensure DCMTK libraries can be found on system without setting DCMTK_DIR
  Improve FindDCMTK.cmake documentation
Merge branch '403-relocatable-ctkconfig-usectk' into pu

* 403-relocatable-ctkconfig-usectk:
  Add support for install-able and relocatable package.
  Remove explicit setting of CMake policies.
  Explicitly set default value for CTK_INSTALL_QTPLUGIN_DIR
  Remove commented code and unused vars from CTKConfig / CTKGenerateCTKConfig
  Set plugin install rules for target and include files
  Install CTK's *.cmake files
  By default, do not install application headers
  Ensure "CMAKE_INSTALL_PREFIX" is properly passed to inner project.
  Ensure CTK_INSTALL_QTPLUGIN_DIR is properly set
Merge branch '405-support-dcmtk-360' into pu

* 405-support-dcmtk-360:
  Add DCMTK 3.6.0 support to ctkDICOMUtil.cpp