Implement the infrastructure for feature-test macros. Very few actual feature test macros, though. Reviewed as: https://reviews.llvm.org/D51955

llvm-svn: 342073
diff --git a/libcxx/include/functional b/libcxx/include/functional
index b91fd75..41beb40 100644
--- a/libcxx/include/functional
+++ b/libcxx/include/functional
@@ -487,6 +487,7 @@
 #include <memory>
 #include <tuple>
 #include <utility>
+#include <version>
 
 #include <__functional_base>
 
@@ -2340,8 +2341,6 @@
 
 #if _LIBCPP_STD_VER > 14
 
-#define __cpp_lib_invoke 201411
-
 template <class _Fn, class ..._Args>
 result_of_t<_Fn&&(_Args&&...)>
 invoke(_Fn&& __f, _Args&&... __args)