diff --git a/js/src/js-config.h.in b/js/src/js-config.h.in
index 8372623..498ea96 100644
--- a/js/src/js-config.h.in
+++ b/js/src/js-config.h.in
@@ -68,4 +68,8 @@
 #undef MOZJS_MAJOR_VERSION
 #undef MOZJS_MINOR_VERSION
 
+/* Controls API in UniquePtrExtensions.h. */
+#undef XP_UNIX
+#undef XP_WIN
+
 #endif /* js_config_h */