Make QtWebKit configure report the right configuration

Debug and release configurations are mutually exclusive, so only the
last one defines the configuration. QtWebKit was however reporting
both if both were set producing confusion output.

Change-Id: If26e4ee02c922fbb880197379c6967c85746b482
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>