handling redirect download/on the fly packaging URLs?

[Georg Schwarz <georg.schwarz%freenet.de@localhost>]

I'm trying to add a package which needs to fetch its source (in  
tar.gz format) from http://github.com/philpraxis/ss7calc/tarball/master
Of course I could add the URL to which one gets redirected, but this  
is insufficent since the tarball is only created on the fly when the  
download URL in requested.
What's the best way to handle this with pkgsrc?

--
Georg Schwarz    http://home.pages.de/~schwarz/
georg.schwarz%freenet.de@localhost  +49 151 11559652