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_view b/include/string_view
index 6377aeb..55dce72 100644
--- a/include/string_view
+++ b/include/string_view
@@ -178,6 +178,7 @@
 #include <iterator>
 #include <limits>
 #include <stdexcept>
+#include <version>
 #include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)