use -Wunused-parameter with gcc for consistency with MSVC and other compilers which warn about this; fix the uncovered warnings in wxGTK

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@49376 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/statbox.cpp

@@ -36,7 +36,7 @@ extern "C" {
 static void
 gtk_frame_size_allocate (GtkWidget     *widget,
                          GtkAllocation *allocation,
-                         wxStaticBox *p)
+                         wxStaticBox * WXUNUSED(box))
 {
     GtkFrame *frame = GTK_FRAME (widget);