Enhanced DCMTK CMake exports for external projects.

The files DCMTKConfig.cmake, DCMTKConfigVersion.cmake and
DCMTKTargets.cmake are exported by CMake to the DCMTK build tree as
as well as to the DCMTK installation directory. The files can be found
and evaluated by other CMake-based projects using CMake's find_package
command (in "Config" mode). The files describe DCMTK executables and
libraries as well as DCMTK version information, which then can be
facilitated in an automatic way by external projects to configure and
perform their own build process. Thanks to Jean-Christophe Fillion-Robin
from Kitware for the original patch.