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/unordered_set b/include/unordered_set
index 9b8560d..a219fa6 100644
--- a/include/unordered_set
+++ b/include/unordered_set
@@ -338,6 +338,7 @@
 #include <__hash_table>
 #include <__node_handle>
 #include <functional>
+#include <version>
 
 #include <__debug>