CXXFeatures: allow C++11 flag to be vary if compiler has unexpected features

This is especially useful for Clang or the Clang g++ plugin.
Merge topic 'cxx11' into next

b3c778a CXXFeatures: allow C++11 flag to be vary if compiler has unexpected features
CXXFeatures: try to prevent failure with g++ 2.9
Merge topic 'cxx11' into next

9dad475 CXXFeatures: try to prevent failure with g++ 2.9