COMP: FindDCMTK - Exclude -D from DCMTK_DEFINITIONS for easier re-use

Since target properties like COMPILE_DEFINITIONS
or INTERFACE_COMPILE_DEFINITIONS do not expect definition to include "-D",
this commit exclude it from definition list associated
with DCMTK_DEFINITIONS.
Merge branch '382-fix-find-dcmtk-issue'

* 382-fix-find-dcmtk-issue:
  COMP: FindDCMTK - Exclude -D from DCMTK_DEFINITIONS for easier re-use