STYLE: Allow system installed expat library with ITK_USE_SYSTEM_EXPAT=ON

Change-Id: I848f006063f27de4cdaf35e3a84de102caadb39f
Merge topic 'ExternalExapt_ITK_USE_EXTERNAL_EXPAT'

ad260804 STYLE: Allow system installed expat library with ITK_USE_SYSTEM_EXPAT=ON