bug#21968: Apparent regression on the undo command

[Eli Zaretskii]

> From: John Wiegley <jwiegley@gmail.com>
> Date: Mon, 14 Dec 2015 12:04:59 -0800
> Cc: David Reitter <david.reitter@gmail.com>, 21968@debbugs.gnu.org
> 
> I just built emacs-25, but most actions that I take give me errors like the
> following:
> 
>     Symbol’s function definition is void: undo-auto-amalgamate
> 
> Have I omitted to do something?

I don't know, but that function is in simple.el.  Did you build in a
clean directory?