Associate DCMTK_DEFINITIONS to CTKDICOMCore

Following commit 12c052ff1, FindDCMTK.cmake has been updated to
detect if HAVE_CONFIG_H should be passed when compiling against
older version of DCMTK packaged on Debian.

To keep build system of project depending on CTK simple, instead of
manually adding the definition for all target depending
on CTKDCMTKCore, the definition is added as a target
property INTERFACE_COMPILE_DEFINITIONS to CTKDCMTKCore in CTKConfig.cmake.

That way, all target depending on the imported target CTKDCMTKCore
will automatically specify the -DHAVE_CONFIG_H flag.

Since the property INTERFACE_COMPILE_DEFINITIONS has been added
in CMake 2.8.11, it will still be required to specify the definition
manually in project using older version of CMake.

See #413
Merge branch '413-associated-dcmtk-definitions-to-ctkdicomcore'

* 413-associated-dcmtk-definitions-to-ctkdicomcore:
  Associate DCMTK_DEFINITIONS to CTKDICOMCore

Fixes #413