What does "Changing `byte-compile-dest-file'" mean?

[Alan Mackenzie]

Hello, Emacs.

I'm trying to debug a failing bootstrap.  Central to the failure is the
error message:

    "Changing `byte-compile-dest-file' is obsolete (as of 23.2);
    set `byte-compile-dest-file-function' instead."

..  The error message comes from emacs-lisp/bytecomp.el L. 171.

But what does it mean?  In what respect is byte-compile-dest-file being
changed?  Is the message talking about the variable or the function of
that name?  (I'm guessing the function.)  Is the entity being altered in
some fashion, or is it getting replaced by something else?

What is causing the error?  It seems that if a function called
byte-compile-dest-file is already defined when bytecomp.el first gets
loaded, this triggers the error.  But what causes that circumstance?

There are no clues in the comments around that part of bytecomp.el.  Just
that it might have something to do with early versions of automake.
(Which we don't use, do we?)

Help!

Thanks in advance!

-- 
Alan Mackenzie (Nuremberg, Germany).