Backport G_GNUC_MALLOC fixes from #1465 to glib-2-56
Trivial backport with two changes:
- Change the reference to ‘GLib 2.58’ in the
G_GNUC_MALLOC
documentation to ‘GLib 2.56.3’ - Drop the changes to
grcbox.h
, which didn’t existing 2.56.x
Closes #1465 (closed)