The option defaults to 'auto' which keeps the current selection behavior, but
also allows user to override the choice.
Also make the chosen backend is reported to the code via CPP defines.
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
