Features: activate C++11 support for Clang 3.1+
This enables building targets that use the CXX_STANDARD
target property to select the C++11 standard when using Clang < 3.4. I tested this with Clang 3.3.
I suspect that the same changes for Clang 2.9 would equally well but I don't have a compiler in that version range to check with.