The dialog is shown when the cmake configuration changes.
The user can either apply the changes or reject them. To determine the
decision of the user the return value of the dialog is evaluated.
This is wrong because the dialog uses custom buttons (see documentation
of QMessageBox::exec). As a consequence the configuration is never
applied.

Use QMessageBox::clickedButton to determine the user decision.
Additionally change the role of the apply button from AcceptRole to
ApplyRole as this better matches its intention.

Change-Id: I1d2d1fb7186dcc8d789c192c51bb34111eb84ee5
Task-number: QTCREATORBUG-18292
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>