meson: add a dummy build option for the mir backend that errors out

Maybe this will motivate someone to implement it.
4 jobs for gtk3-meson-finish in 12 minutes and 22 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #327107
fedora-autotools

00:12:20

passed #327108
fedora-meson

00:07:09

manual #327110
win32 allowed to fail manual
msys2-mingw32-autotools
passed #327109
win32
msys2-mingw32-meson

00:04:05