Enable C++11 on all platforms except Mac/release

This allows us to profit from the Qt extensions for C++11, and also
makes sure e.g. QTC_OVERRIDE is consistenly checked. We can't enable
it (yet) for Mac/release though to be able to deploy to 10.6 ..

Change-Id: Icd26ab5f971532ff10a892221c653987026ac88d
Reviewed-by: Eike Ziller <eike.ziller@digia.com>