Enable C++11 on OSX when using Clang

This means we have to bump the deployment target to Lion (10.7), as the
LLVM 'libc++' C++ standard library does not support Snow Leopard (10.6).

For iOS the deployment target has to be bumped from 4.3 to 5.0, but we
don't enable C++11 by default yet as it's not tested enough on iOS.

Users who wish to deploy to 10.6 need to build their own Qt,
passing -no-c++11 to configure.

Change-Id: I7b5d20ab002db889d1091a4b7ff600f62caa7f06
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>