Features: Activate C++ 17 support for AppleClang 6.1+

Modules/Compiler/AppleClang-CXX.cmake

@@ -22,6 +22,11 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.1)
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
 endif()
 
+if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 6.1)
+  set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std=c++1z")
+  set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++1z")
+endif()
+
 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
   if (NOT CMAKE_CXX_COMPILER_FORCED)
     if (NOT CMAKE_CXX_STANDARD_COMPUTED_DEFAULT)
@@ -38,6 +43,9 @@ endif()
 macro(cmake_record_cxx_compile_features)
   set(_result 0)
   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
+    if(_result EQUAL 0 AND CMAKE_CXX17_STANDARD_COMPILE_OPTION)
+      _record_compiler_features_cxx(17)
+    endif()
     if(_result EQUAL 0 AND CMAKE_CXX14_STANDARD_COMPILE_OPTION)
       _record_compiler_features_cxx(14)
     endif()