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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@177297 91177308-0d34-0410-b5e6-96231b3b80d8
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