Removed raw references to __APPLE__; now just check to see if it is defined.

llvm-svn: 177297
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b56e8587aff9d8766a673f51b2d9c525e1535d84
diff --git a/include/iterator b/include/iterator
index 59e3390..3b078a2 100644
--- a/include/iterator
+++ b/include/iterator
@@ -317,7 +317,7 @@
 #include <type_traits>
 #include <cstddef>
 #include <iosfwd>
-#if __APPLE__
+#ifdef __APPLE__
 #include <Availability.h>
 #endif