As per https://gitlab.gnome.org/GNOME/glib/-/issues/3421#note_2206315

:

It seems like there’s agreement that glib_debug should be enabled for
developers and disabled for distros; and it also seems like there’s no
reliable way to figure this out magically (because not everyone ties
things to `-Dbuildtype=*`). So, we’re left with forcing some group of
people to manually set the value of `glib_debug`. There are more
developers/contributors than there are distros, and distros are more
likely to notice an accidentally-slow GLib package than developers are
likely to notice an accidentally-not-asserting-hard-enough local build,
so let’s say:

The default should be `-Dglib_debug=enabled`, and distros should probably
all override that to `-Dglib_debug=disabled`.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3421