(un)setting environment variables from a Portfile

[René J.V. Bertin]

Hi,

I see MacPorts provides a set of keywords to add or delete env. variables to or from specific phases. Is there a (Tcl) command one can use to set or unset variables at an appropriate point so it's defined (or missing) appropriately for all subsequent steps?

If not, can one use *.env-delete to unset a variable without knowing its value?

Case at hand: I want to make sure QTDIR is not defined when configuring and building (etc) port:qt*-mac .

Thanks,
René