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-fixes in 1 minute and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #7369810
debian-buster

00:00:08

 
  Build And Test
passed #7369811
meson

00:01:37

passed #7369812
meson-noglamor

00:01:36