bug#20405: Can't build master

[Paul Eggert]

If it just started happening around April 20, and it started happening in both 
24.5 and 25-git, then it's probably something to do with your system rather than 
a change we made to Emacs, as we haven't changed 24.5 since it was released 
April 10.

But if I'm wrong and a recent change in the master caused the problem, you can 
help by bisecting to find which commit caused the problem; this shouldn't be 
hard if it's a commit around April 20.