I see there has been an attempt to fix this, but in
gnome-utils-2.8.0-1, the post-scripts try to do update-gtk-immodules
on something that on my i686 expands to  i386-redhat-linux-gnu while
gtk2-2.4.10-4 places the file in /etc/gtk-2.0/i686-redhat-linux-gnu
                                               ^
So right now in rawhide the postinstall script is broken.

Unless you built gtk with --target=i686 it should have these files in
/etc/gtk-2.0/i386-redhat-linux-gnu. Please verify this works with
rawhide now and reopen if it doesnt.

I did not build gkt2 with any --target flags, but the bug still exists
in gtk2-2.4.13-5
In the specfile for gtk2, the directory in /etc/gtk-2.0/ in derived
from _host which on my system is defined in /usr/lib/rpm/macros (from
rpm-4.3.1-0.3 )as i686-redhat-linux-gnu.
If this is a gtk2 bug maybe it should be moved to that component.
Also, I do not have sufficient privileges to reopen this bug so maybe
someone else could do so.

I changed it to use _host in rawhide, which isnt without problems but
is more sane.