wayland: Update to 1.18.0

Based on a patch from wiz@, thanks! The main change I made was tricking
autoconf into thinking NetBSD doesn't have posix_fallocate.

The main new features in this release are:

- Add support for the Meson build system (autotools is still supported
  but will be removed in a future release)
- Add API to tag proxy objects to allow applications and toolkits to
  share the same Wayland connection
- Track wayland-server timers in user-space to prevent creating too
  many FDs
- Add wl_global_remove, a new function to mitigate race conditions with
  globals

(nia)