Add DCMTK 3.6.0 support to ctkDICOMUtil.cpp

This commit implements a try_compile checking if DCMTK provides
'dcmtk::log4cplus::Logger', if not fallback to 'log4cplus::Logger'.

Indeed, following DCMTK-3.6.1_20110519 (commit e39d190), log4cplus has
been moved into dcmtk::log4cplus namespace.

See #405