The volatile qualifier was mostly used for historical reasons,
the documentation for `g_once_init_enter()` and `g_once_init_leave()`
has the following to say:
While `location` has a `volatile` qualifier,
this is a historical artifact and the pointer passed to it
should not be volatile.
See also https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719
This is an exact copy from GTK4.
as grabbed by chattys commit 1ed5084fb965908e3ee0304781b0de06479c869b
Slightly adapted for Calls.
Based on GTKs 01bd4cc4e18a1ea697fe61791ba710d0d55e8290