Portfile dependencies relying on specific variant

[Frank Dean]

I've submitted a pull request [1] for a new Portfile, `mod_tile`, but the
checks are failing because this port requires the `mapnik` port to be
installed with a non-default variant, `+postgis`.  Similarly, it requires the
`osm2pgsql` port to have been installed with `+lua`.

[1]: https://github.com/macports/macports-ports/pull/12391

I've specified the dependency with:

    require_active_variants mapnik postgis

If the dependent ports have not already been installed with the correct
variant, simply performing a `port install` on `mod_tile` fails due to this
dependency check, unless the install command includes the variant specifiers.

1.  How should I handle this so that the automated builds complete
    successfully?

2.  How can I improve the user experience in the same scenario?


Thanks,

Frank Dean