NixOS/nixpkgs — pull request #1880 by pSub (2014-03-04 16:43:12+00:00)
Title: Use feh only as a fallback if the xserver is actually enabled.

Otherwise feh is installed even though no xserver is available. It would be better to check in addition if the desktop manager is enabled (so that feh isn't installed if no desktop manager is enabled at all), but I don't think there is a reliable way to check this.