X.org is installed in /usr/X11R7.
Attached patch covers this in the x11 check.
Xen has no direct X11 dependency
(it's inherited from SDL and SDL is optional for qemu-dm)
so warn but don't fail if no X11 is installed.
Fixes build problem on distributions which build their packages (*BSD
and Gentoo).

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>