meson: Simplify build_glamor logic

Don't need the separate glamor_option variable anymore.

Fixes: 274d54d1 "meson: Build Xwayland unconditionally"
3 jobs for xwayland-21.1 in 1 minute and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed #7344338
debian-buster

00:00:09

 
  Build And Test
passed #7344339
meson

00:01:31

passed #7344340
meson-noglamor

00:01:31