The download url http://research.warnes.net/~warnes/fpconst/fpconst-0.7.1.tar.gz results in a 404. Should be: http://research.warnes.net/downloads/fpconst/fpconst-0.7.1.tar.gz
Both links do work outside emerge. There's something else wrong. The official homepage: http://research.warnes.net/projects/rzope/fpconst/ points to the first one, so I've manually added fpconst-0.7.1.tar.gz to distfiles. It will be available on local distfiles mirrors soon. I'll leave the URL like this for now, until I find what's wrong with it. Thanks for the report.