Maybe because libX11<=1.3.6 had an xcb use flag, but it was removed in 1.4.1
Code:
# Copyright 1999-2010 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: /var/cvsroot/gentoo-x86/x11-libs/libX11/libX11-1.3.6.ebuild,v 1.1 2010/09/21 08:11:11 scarabeus Exp $
EAPI=3
inherit xorg-2 toolchain-funcs flag-o-matic
DESCRIPTION="X.Org X11 library"
KEYWORDS="~alpha ~amd64 ~arm ~hppa ~ia64 ~mips ~ppc ~ppc64 ~s390 ~sh ~sparc ~x86 ~x86-fbsd"
IUSE="doc ipv6 test +xcb"
RDEPEND=">=x11-libs/xtrans-1.2.3
x11-proto/kbproto
>=x11-proto/xproto-7.0.13
xcb? ( >=x11-libs/libxcb-1.1.92 )
!xcb? (
x11-libs/libXau
x11-libs/libXdmcp
)"
DEPEND="${RDEPEND}
doc? (
app-text/ghostscript-gpl
sys-apps/groff
)
test? ( dev-lang/perl )
x11-proto/xf86bigfontproto
!xcb? (
x11-proto/bigreqsproto
x11-proto/xcmiscproto
)
x11-proto/inputproto
x11-proto/xextproto"
Code:
# Copyright 1999-2011 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: /var/cvsroot/gentoo-x86/x11-libs/libX11/libX11-1.4.1.ebuild,v 1.9 2011/03/05 17:53:20 xarthisius Exp $
EAPI=3
inherit xorg-2 toolchain-funcs flag-o-matic
DESCRIPTION="X.Org X11 library"
KEYWORDS="alpha amd64 arm hppa ia64 ~mips ppc ppc64 s390 sh sparc x86 ~ppc-aix ~x86-fbsd ~x64-freebsd ~x86-freebsd ~x86-interix ~amd64-linux ~x86-linux ~ppc-macos ~x64-macos ~x86-macos ~sparc-solaris ~sparc64-solaris ~x64-solaris ~x86-solaris ~x86-winnt"
IUSE="doc ipv6 test"
RDEPEND=">=x11-libs/libxcb-1.1.92
x11-libs/xtrans
x11-proto/xf86bigfontproto
x11-proto/inputproto
x11-proto/kbproto
x11-proto/xextproto
>=x11-proto/xproto-7.0.13"
DEPEND="${RDEPEND}
doc? ( app-text/xmlto )
test? ( dev-lang/perl )"