looking at http://tinderbox.dev.gentoo.org/misc/rindex/dev-util/gtk-doc (which should be empty), I've dig into the ebuild to find that it looks very much like RDEPEND was inverted with DEPEND
Ah yes if you mean dev-util/pkgconfig that should be a ddep not a rdep :) Oopsie will look to correct that ASAP.
*** Bug 220385 has been marked as a duplicate of this bug. ***
Ok finally got a chance to invert them. My bad, thanks for reporting :)