Reapply 135035 with proper conditional inclusion, hopefully solving
issues with it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@135246 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config b/include/__config
index a5afbca..2610d81 100644
--- a/include/__config
+++ b/include/__config
@@ -286,6 +286,10 @@
 #endif
 
 #ifdef __APPLE__
+#define _LIBCPP_LOCALE__L_EXTENSIONS 1
+#endif
+
+#ifdef __APPLE__
 #define _LIBCPP_STABLE_APPLE_ABI
 #endif