Fixed detection of C++11 features (#224)

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.

dlib/cmake_utils/use_cpp_11.cmake

@@ -52,15 +52,16 @@ if (CMAKE_VERSION VERSION_LESS "3.1.2")
    endif()
 else()
    # Set a flag if the compiler you are using is capable of providing C++11 features.
-   if (";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_rvalue_references;" AND
-       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_variadic_templates;" AND
-       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_lambdas;" AND
-       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_defaulted_move_initializers;" AND
-       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_delegating_constructors;" AND
-       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_thread_local;" AND
-       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_constexpr;" AND
-       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_decltype_incomplete_return_types;" AND
-       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_auto_type;")
+   get_property(cxx_features GLOBAL PROPERTY CMAKE_CXX_KNOWN_FEATURES)
+   if (";${cxx_features};" MATCHES ";cxx_rvalue_references;" AND
+       ";${cxx_features};" MATCHES ";cxx_variadic_templates;" AND
+       ";${cxx_features};" MATCHES ";cxx_lambdas;" AND
+       ";${cxx_features};" MATCHES ";cxx_defaulted_move_initializers;" AND
+       ";${cxx_features};" MATCHES ";cxx_delegating_constructors;" AND
+       ";${cxx_features};" MATCHES ";cxx_thread_local;" AND
+       ";${cxx_features};" MATCHES ";cxx_constexpr;" AND
+       ";${cxx_features};" MATCHES ";cxx_decltype_incomplete_return_types;" AND
+       ";${cxx_features};" MATCHES ";cxx_auto_type;")
 
       set(COMPILER_CAN_DO_CPP_11 1)
       # Set which standard to use unless someone has already set it to something