build: ensure that makeinfo ≥ 6.8 checks the @menu structure

See <https://lists.gnu.org/r/bug-texinfo/2023-06/msg00015.html>.

* doc/local.mk (MAKEINFO): New variable.
* cfg.mk (_makefile_at_at_check_exceptions): Update.

cfg.mk

@@ -52,7 +52,8 @@ export XZ_OPT = -8e
 old_NEWS_hash = ac6cb1c35890b53e56acbfbb078fcd53
 
 # Add an exemption for sc_makefile_at_at_check.
-_makefile_at_at_check_exceptions = ' && !/^cu_install_prog/ && !/dynamic-dep/'
+_makefile_at_at_check_exceptions = \
+  ' && !/MAKEINFO/ && !/^cu_install_prog/ && !/dynamic-dep/'
 
 # Our help-version script is in a slightly different location.
 _hv_file ?= $(srcdir)/tests/misc/help-version

doc/local.mk

@@ -25,6 +25,10 @@ doc_coreutils_TEXINFOS = \
   doc/fdl.texi \
   doc/sort-version.texi
 
+# The customization variable CHECK_NORMAL_MENU_STRUCTURE is necessary with
+# makeinfo versions ≥ 6.8.
+MAKEINFO = @MAKEINFO@ -c CHECK_NORMAL_MENU_STRUCTURE=1
+
 # The following is necessary if the package name is 8 characters or longer.
 # If the info documentation would be split into 10 or more separate files,
 # then this is necessary even if the package name is 7 characters long.