Clang: Don't overwrite c++11 compile option value with c++14 flag.

Modules/Compiler/Clang-CXX.cmake

@@ -24,8 +24,8 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 2.1)
 endif()
 
 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
-  set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "-std=c++1y")
-  set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
+  set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++1y")
+  set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
 endif()
 
 set(CMAKE_CXX_STANDARD_DEFAULT 98)