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/string b/include/string
index 162e540..c79164c 100644
--- a/include/string
+++ b/include/string
@@ -510,6 +510,7 @@
 #include <type_traits>
 #include <initializer_list>
 #include <__functional_base>
+#include <version>
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
 #include <cstdint>
 #endif