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

As per #3421 (comment 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 (closed)