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

llvm-svn: 342073
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f56972e22489c92028e5147344ab8b31b0b4fed5
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