that is, save it to config.opt and recall it when -redo is used (and do
not write it again in this case).

a trivial config.status is still created, as having it is very
convenient when shadow-building.

Task-number: QTBUG-38792
Change-Id: I5e9f7374d6bfc60c427cbfd5e9b3e68bfcaae9f2
Reviewed-by: Lars Knoll <lars.knoll@qt.io>