docs: Set -Dglib_debug=enabled by default and document it for distros

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

docs/macros.md

@@ -61,6 +61,10 @@ Quite a bit of additional debugging code is compiled into GLib when this
 macro is defined, and since it is a globally visible define, third-party code
 may be affected by it similarly to `G_DISABLE_ASSERT`.
 
+Some of these checks can be relatively expensive at runtime, as they affect
+every GObject type cast. Distributions are recommended to disable
+`G_ENABLE_DEBUG` in stable release builds.
+
 The additional code executed/compiled for this macro currently includes the
 following, but this is not an exhaustive list:
  - extra validity checks for `GDate`

meson.options

@@ -115,9 +115,9 @@ option('oss_fuzz',
 
 option('glib_debug',
        type : 'feature',
-       value : 'auto',
+       value : 'enabled',
        yield : true,
-       description : 'Enable GLib debug infrastructure (see docs/macros.md)')
+       description : 'Enable GLib debug infrastructure (distros typically want this disabled in production; see docs/macros.md)')
 
 option('glib_assert',
        type : 'boolean',