How to specify a variant in 'depends_lib'

[markd at macports.org]

susanmackay at optusnet.com.au writes:
>I am trying to write a portfile for some software but I want to make sure
>that a particular variant of a dependency is installed. Is there some way
>that I can specify the variant within the portfile that I want to use?
>WOuld something like:
>
>depends_lib   port:otherport+variantname
>
>work? (I'm away from the computer that is doing this at the moment so I
>can't experiment). If not, then I'll have to think of some other way of
>achieving my goal.

There is a characteristic of MacPorts base that can accomplish this,
though it should be used with some care.  Variants are passed to
dependents, so if you install a port with a variant foo, and a dependency
also has a variant foo, then the dependency will get installed with
variant foo.  This is fine for ports where a dependency is only used with
a given port, or rarely used with anything else, or where the dependency
is almost always installed using the variant anyway.  Caveats are that if
a dependency is already installed, it won't help a bit.  Another is that
if the conditions I mentioned do not hold, you may shield the user from
having to learn something they'll only have to learn later anyway.  But
I've done this for a couple of ports where I thought it was warranted. 
See the comments in the server variant of the nedi port for an example.

Mark