Modify config file based on build constants

#if DEBUG
    // do something
#endif

...
<importantSetting>$importantSettingValue$</importantSetting>
...

...
<importantSettingValue>QAvalue</importantSettingValue>
...

...
<importantSettingValue>PRODvalue</importantSettingValue>
...

XmlPreProcess.exe app.config qavalues.xml