ports/devel/glib20: build failure due to race

From: Eric van Gyzen <eric_at_vangyzen.net>
Date: Wed, 26 Jul 2023 01:56:45 UTC
This merge request fixes a race condition that makes the build of devel/glib20 fail.

https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3512

You can make the race more reliable (and therefore test the fix) by adding MAKE_JOBS_UNSAFE=yes and building with samurai instead of ninja.

Cheers,

Eric