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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@342073 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/iterator b/include/iterator
index 9415e0b..aed0525 100644
--- a/include/iterator
+++ b/include/iterator
@@ -418,6 +418,7 @@
 #include <type_traits>
 #include <cstddef>
 #include <initializer_list>
+#include <version>
 #ifdef __APPLE__
 #include <Availability.h>
 #endif