handle variant dependency

[Jeremy Lavergne]

> The py25-gtk, py26-gtk, py27-gtk depend on cairo +x11.
> 
> I recall that the suggestion to test for a variant specific file and error if it is [not] present. In this case is there anything that could/would prevent a user from upgrading the variant away with --enforce-variants?

You can avoid that creating a dummy default variant on your port to match. If the variant is disabled, then you can error out like before. Then --enforce-variants won't harm anything unless they run it explicitly against a specific port rather than all (since it'll alert the user when it goes to rebuild your port).