Conditionally wrap the changes from r134781.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@134783 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config b/include/__config
index b138510..239f0a2 100644
--- a/include/__config
+++ b/include/__config
@@ -285,4 +285,8 @@
 #define __has_feature(__x) 0
 #endif
 
+#ifdef __APPLE__
+#define _LIBCPP_STABLE_APPLE_ABI
+#endif
+
 #endif  // _LIBCPP_CONFIG