Skip Ubuntu properties

[waywardmonkeys]

On Ubuntu, we generate properties like these:

define property-setter ubuntu-no-proxy :: <C-boolean> on <GtkWindow> end;
define property-getter ubuntu-no-proxy :: <C-boolean> on <GtkWindow> end;
define property-setter ubuntu-almost-fixed-height-mode :: <C-boolean> on <GtkTreeView> end;
define property-getter ubuntu-almost-fixed-height-mode :: <C-boolean> on <GtkTreeView> end;