xserver_xorg-server: add explicit handling for xwayland

It's normally autodetected, which can lead to unexpected/unhandled
results.
configure checks for libdrm, libepoxy and wayland, however a proper
libxcomposite check is missing thus it can lead to build failure under
some odd conditions.
There's no autobuilder failure to quote here, however there's a mailing
list post that exemplifies this:
http://lists.busybox.net/pipermail/buildroot/2016-May/161793.html



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>