This is probably because I'm running Break My Gentoo, but I thought I'd share the patch in case anyone else gets the same problem Reproducible: Always Steps to Reproduce:
Created attachment 51983 [details, diff] Fix deprecated usage of libgnome/gnome-i18n.h
Created attachment 51984 [details] Fixed ebuild
BMG bugs should be posted to their bugzilla, not here. Standard procedure is INVALID. Thanks.
The failure is in a normal Gentoo build. I may well be getting in because I have BMG packages installed. Once I fixed it I didn't investigate further.
libgtkhtml is using a deprecated header that was removed in the GNOME 2.10 release cycle; hence, BreakMyGentoo brings this bug to light. It is, nevertheless, a problem with a package in the Gentoo tree and this is the correct fix. I recommend that the patch be applied to CVS.