Detecting C++11 capabilities did not work on OSX 10.11.6, XCode 7.3.1, Apple LLVM 7.3.0, cmake 3.6.1. First check revealed that CMAKE_CXX_KNOWN_FEATURES was empty, so subsequent matches failed. After reading http://public.kitware.com/pipermail/cmake/2014-November/059182.html changed handling to fetch property instead of relying on variable. Works on my machine.