fix qbs clang build

The clang flags for C++11 were incomplete.
The linker flag -lc++abi isn't needed.
On OS X the minimum OS version must be set to 10.7.

This amends 7309b6a2d9a30cc6be7087c12aec2ada65de7b36.

Change-Id: I71c1b8b3f9901ecc8f04106874f5697cfaa47a26
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>