zbar depends on =dev-libs/glib-2* rather than on dev-libs/glib:2, causing the conflict if glib-9999 is available. The ebuild is already EAPI 4, and already uses a slot depend for x11-libs/gtk+:2, so a simple s/=dev-libs/glib-2*/dev-libs/glib:2/ should do.
fixed in next revbump, thanks
+*zbar-0.10-r3 (19 Apr 2013) + + 19 Apr 2013; Michael Weber <xmw@gentoo.org> +zbar-0.10-r3.ebuild: + Fix glib slot dep (bug 444644, thanks James Cloos) +