QtCreator thinks that the qmake project configuration has been changed when you supply something like CONFIG+="foo bar" (with space) as additional qmake argument. This means that upon every "Build" and "Run" command, it runs qmake, even if nothing has changed, which for a large project can take significant time.

If you instead supply CONFIG+=foo CONFIG+=bar as two separate arguments, it works without issue.

I can reproduce this with both 4.0.2 and 3.6.1

See also my initial question in the forum: https://forum.qt.io/topic/68578/qtcreator-reruns-qmake-every-time-i-run-or-build