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/mutex b/include/mutex
index 52e39b0..9c55c7c 100644
--- a/include/mutex
+++ b/include/mutex
@@ -194,6 +194,7 @@
 #ifndef _LIBCPP_CXX03_LANG
 #include <tuple>
 #endif
+#include <version>
 #include <__threading_support>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)