Index: lib/Basic/Targets.cpp
===================================================================
--- lib/Basic/Targets.cpp
+++ lib/Basic/Targets.cpp
@@ -856,6 +856,8 @@
     // Follow g++ convention and predefine _GNU_SOURCE for C++.
     if (Opts.CPlusPlus)
       Builder.defineMacro("_GNU_SOURCE");
+
+    DefineStd(Builder, "unix", Opts);
   }
 
   // As an optimization, group static init code together in a section.