dev-libs/weston-9999: migrate to meson

dev-libs/weston-9999: migrate to meson

Upstream after v6 won't support autotools:
https://lists.freedesktop.org/archives/wayland-devel/2019-February/040080.html

Also bump the wayland dependency, because current master matches v6
beta, and weston 6 will be released alongside wayland 1.17. Before the
release, >=1.17 resolves to 9999, which is how I'm testing it.

Signed-off-by: makepost <makepost@firemail.cc>
Closes: https://github.com/gentoo/gentoo/pull/11326
Signed-off-by: James Le Cuirot <chewi@gentoo.org>