https://github.com/carlobaldassi/gimp-lqr-plugin/commit/ae3464a82e1395fc577cc94999bdc7c4a7bb35f1

Index: src/interface_aux.c
--- src/interface_aux.c.orig
+++ src/interface_aux.c
@@ -48,11 +48,11 @@ static void callback_dialog_aux_response (GtkWidget * 
 
 gint dialog_aux_response = GTK_RESPONSE_OK;
 
-PlugInUIVals *ui_state;
-PlugInVals *state;
-PlugInDialogVals *dialog_state;
+extern PlugInUIVals *ui_state;
+extern PlugInVals *state;
+extern PlugInDialogVals *dialog_state;
 
-GtkWidget *dlg;
+extern GtkWidget *dlg;
 
 /***  Public functions  ***/