COMP: Ensure FindDCMTK module works with and without DCMTKConfig.cmake file

In the case the provided DCMTK build or install tree doesn't have a
DCMTKConfig.cmake file, the first call to "find_package" should fail
quietly so that it doesn't prevent the rest of discovery process from working
as expected.

Adapted from commontk/CTK@74b4b07b

Change-Id: I2edb9f8becf46aa208f56a909bc1ba84052db374
Merge topic 'fix-building-against-system-dcmtk'

14fc275 COMP: Ensure FindDCMTK module works with and without DCMTKConfig.cmake file