these variables have no legitimate use when building qt itself, but have
great potential to wreak havoc.

Task-number: QTBUG-60016
Change-Id: I161837463443af82d48145e75952fa529212fe75
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>