BaseQtVersion: Make sure qmake variables are up to date.

Otherwise, qmake properties can be wrongly read as empty.

Change-Id: If63a306664758c01529b437fca30f5222b75f02f
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>