CMake: Allow to unset configuration values

Change-Id: I649323e3c2cc51ea69dd7e216f30eeb653f3873b
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildsettingswidget.h

@@ -78,6 +78,7 @@ private:
     QPushButton *m_addButton;
     QMenu *m_addButtonMenu;
     QPushButton *m_editButton;
+    QPushButton *m_unsetButton;
     QPushButton *m_resetButton;
     QCheckBox *m_showAdvancedCheckBox;
     QPushButton *m_reconfigureButton;