Since bison has recently been removed from the system profile, webkit-gtk will fail to build if bison is not installed.
Please provide full build.log showing the error
Created attachment 280947 [details] build.log build.log attached
Created attachment 280949 [details] emerge --info
+ 27 Jul 2011; Pacho Ramos <pacho@gentoo.org> webkit-gtk-1.4.2-r200.ebuild, + webkit-gtk-1.4.2-r300.ebuild: + Add sys-devel/bison DEPEND, bug #376291 by Albert W. Hopkins. +