<Multi_key> is undefined

[David Kastrup <dak@gnu.org>]

After upgrading to Ubuntu 14.04 (and using Cinnamon), I can no longer
use <Multi_key>.  Emacs flashes at me and displays "<Multi_key> is
undefined".  This happened with a binary compiled before the upgrade,
and it also happens after I bootstrapped and built Emacs from a clean
directory right now.

The toolkit used is GTK3 (without toolkit scroll-bars, but I should be
surprised if that made a difference).

Any pointers?  Using input modes is quite less convenient than relying
on Multi_key for me.  What should I be testing?  What info might help
further pinpointing this?

I think it's important to get this fixed before the next release.  Many
programmers use an American keyboard layout rather than a local one
(because of characters like []{}|) and rely on Multi_key for their
non-English correspondence and text processing.

Now this is clearly related to upgrading the base system, but things
like the terminal window continue working with Multi_key just fine.

-- 
David Kastrup