Ignoring unknown mode `GNUmakefile-mode'

[Dan Hitt <dan.hitt@gmail.com>]

I have a file whose name is not 'makefile', nor 'Makefile', nor 'GNUmakefile'.

If i visit it using emacs (v 24.5.1, xubuntu 16.04), the modeline
correctly indicates GNUmakefile.

But if i add
# -*- GNUmakefile -*-
at the top of the file, i get a message:
    Ignoring unknown mode `GNUmakefile-mode'.

This makes me very uneasy, because i don't know how it is detecting
the mode, and if i mess up the file, it may no longer recognize it as
a GNUmakefile.  Also, i don't know if other versions of emacs will be
able to recognize that it is a GNUmakefile.

Note that i can explicitly set the mode with a line like
# -*- Makefile -*-

That does not generate any warnings, but the mode line indicates
'Makefile' rather than 'GNUmakefile', which is not really correct
since it uses a bunch of GNUmakefile specific things.

So . . . is there some way i can instruct emacs, in the text of the
file, that i want it to be treated as a GNUmakefile?

TIA for any clues!

dan

PS: i googled for the line in the subject, and was very surprised not
to get a single match.