When QMAKE_* variable assignments were passed to the configure line
they would cause the current contents of the private pro output to be
overwritten. This would cause anything added to it before the QMAKE_*
variable assignments to be parsed to be lost.

Change-Id: Idcb8cad5f07cbb96b4da204384f5618b95b375b0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>