Using env variable : OZONE_WAYLAND_USE_XDG_SHELL

(name is inspired from Qt and EFL implementations of xdg-shell)

So far xdg-shell is an extension to weston,
let's make it available at runtime
but make sure to have xdg-shell enabled also at buildtime.

Without this env variable defined (ie: in /etc/profile),
if no other alt shell is selected
then wl-shell will be used as (mandatory) fallback.

Signed-off-by: Philippe Coval <philippe.coval@open.eurogiciel.org>
Merge pull request #271 from rzr/master

Using env variable : OZONE_WAYLAND_USE_XDG_SHELL