Features: Use -std=c++17 for C++ 17 on Clang 5.0+

Clang 5.0 added support for the `-std=c++17` flag.

Modules/Compiler/Clang-CXX.cmake

@@ -32,7 +32,10 @@ if(NOT "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
     set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
   endif()
 
-  if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)
+  if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.0)
+    set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std=c++17")
+    set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++17")
+  elseif (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)
     set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std=c++1z")
     set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++1z")
   endif()