Since https://codereview.chromium.org/2788063002


libxml2 is expected to have disabled catalog
support. Prevent crashes of render process
and do not link against system libxml2 if it is
compiled with catalog support.

Task-number: QTBUG-66488
Change-Id: I2d922929878262c555802336841536eba1294db2
Reviewed-by: Kai Koehne <kai.koehne@qt.io>