```cmake
set(DOCUMENTATION "This module contains the third party <a
href=\"http://sourceforge.net/projects/gdcm/\">GDCM</a> library.
Grassroots DiCoM is a C++ library for DICOM medical files.")

itk_module(ITKGDCM
  DEPENDS
    ITKZLIB
    ITKExpat
    ITKOpenJPEG
  DESCRIPTION
    "${DOCUMENTATION}"
)
```

Exclude ITKGDCM module when system GDCM is ON.
user
```cmake
set(DOCUMENTATION "This module contains the third party <a
href=\"http://sourceforge.net/projects/gdcm/\">GDCM</a> library.
Grassroots DiCoM is a C++ library for DICOM medical files.")

if(ITK_USE_SYSTEM_GDCM)
  itk_module(ITKGDCM
    DESCRIPTION
      "${DOCUMENTATION}"
    EXCLUDE_FROM_ALL
    )
else()
  itk_module(ITKGDCM
    DEPENDS
      ITKZLIB
      ITKExpat
      ITKOpenJPEG
    DESCRIPTION
      "${DOCUMENTATION}"
  )
endif()
```
assistant