Makeinfo versions < 6.7 detected mistakes in the @menu structure of TeXinfo
input. Makeinfo versions ≥ 6.8 don't do this any more by default. They need
an extra option, for this validation to happen. See
<https://lists.gnu.org/archive/html/bug-texinfo/2023-06/msg00015.html>.

Since doc/diffutils.texi has a hand-written @menu, it should use this extra
option.

Here's a patch to that effect.

There are two possibilities for adding the extra option:
  - in the MAKEINFO variable, where it has an effect on both "makeinfo"
    and "makeinfo --html",
  - in the AM_MAKEINFOFLAGS variable, where it has an effect on "makeinfo"
    only.
Since some maintainers may check their documentation edits only by regenerating
the HTML-formatted documentation, the first choice is preferrable.

Tested by running
  touch doc/diffutils.texi ; (cd doc && make diffutils.info V=1)
and
  make sc_makefile_at_at_check