AppleClang: Use modern C++14 standard flags for Apple Clang 6.1

The Apple Clang 6.1 compiler that comes with Xcode 6.3 is aware of the
modern -std=c++14 and -std=gnu++14 flags, so use them instead of the
"1y" flags.

Suggested-by: [email protected]

Modules/Compiler/AppleClang-CXX.cmake

@@ -13,7 +13,10 @@ if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
   set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++11")
 endif()
 
-if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.1)
+if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 6.1)
+  set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++14")
+  set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++14")
+elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.1)
   # AppleClang 5.0 knows this flag, but does not set a __cplusplus macro greater than 201103L
   set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++1y")
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")