breakage from r99712

[Jeremy Lavergne]

>   Am I reading this wrong...
> 
> http://trac.macports.org/ticket/126
> 
> It seems to imply that MacPorts doesn't allow a Portfile to enforce a variant in depends_lib?

We can get around this a few ways:
 * PortGroup active_variants
 * subports of tcl/tk

> This seems like a horrible situation and we have just shifted the pain and suffering around.
> The current situation allows packages like pymol to become randomly broken and will totally
> confound end-users.

Sounds like switching the variants to subports will mitigate the issue.