kde-base/klipper-4.8.1 (probably 4.7.4 too, didn't test) lacks USE flags for enabling/disabling various stuff like prison, GoogleGadgets, and PAM. It auto-detects them at build time, and if found, links against them. For example, if media-libs/prison is installed on the system, kde-base/klipper cannot be told not to use it.
Created attachment 307375 [details, diff] klipper-4.8.1.ebuild.patch klipper is part of kde-workspace and therefore inherits a number of configure checks that aren't actually used. Prison however is indeed an automagic dependency, and attached is a patch correcting this.
Thank you all. Fixed in =kde-base/klipper-4.8.1. Sync in some hours to get the changes. + 02 Apr 2012; Johannes Huber <johu@gentoo.org> klipper-4.8.1.ebuild, + metadata.xml: + Fix automagic dependency on media-libs/prison by Michael (kensington) + <gentoo@scribeofthenile.com> wrt bug #410443.