Fix undefined variable
diff --git a/src/couch/rebar.config.script b/src/couch/rebar.config.script
index 9ba8f18..5fd94ff 100644
--- a/src/couch/rebar.config.script
+++ b/src/couch/rebar.config.script
@@ -88,6 +88,8 @@
     {"PACKAGE_VERSION", "\"" ++ Version ++ "\""}
 ].
 
+APP_DIR = filename:dirname(SCRIPT).
+
 CouchJSConfig = case SMVsn of
     "78" ->
         APP_DIR ++ "/priv/couch_js/86/config.h";