it is important that the flags coming from the current qt build appear
first, as otherwise a pre-existing qt installation may interfere with
the build.

the windows configure does not have any of this magic to start with.

Task-number: QTBUG-6351
Change-Id: Iacc1d9b5aa9eed9a5f0513baef9f6c6ffcef0735
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>