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

Index: src/interface_I.c
--- src/interface_I.c.orig
+++ src/interface_I.c
@@ -65,13 +65,13 @@ static void callback_alarm_triggered (GtkWidget * size
 
 gint dialog_I_response = GTK_RESPONSE_OK;
 
-PlugInUIVals *ui_state;
-PlugInVals *state;
-PlugInDialogVals *dialog_state;
-gboolean features_are_sensitive;
+extern PlugInUIVals *ui_state;
+extern PlugInVals *state;
+extern PlugInDialogVals *dialog_state;
+extern gboolean features_are_sensitive;
 InterfaceIData interface_I_data;
 
-GtkWidget *dlg;
+extern GtkWidget *dlg;
 GtkWidget *coordinates;
 
 gulong size_changed = 0;