PET DICOM extension

@jamesobutler this must be something to do with the extensions build system. The extension has not been modified in a very long time.

Found this related discussion: SlicerRT build fails with new DCMTK - #5 by cpinter.

What should I make out of these cdash errors?

https://slicer.cdash.org/viewBuildError.php?buildid=3680439