bug#46837: Makefile dependencies for info files

[Glenn Morris]

Package: emacs
Version: 28.0.50
Tags: help

I suspect that the Makefile dependencies for info files could be
incomplete as of 4825ea3c5d. The symptoms would be info related errors
in parallel builds, and missing files in info/dir.
I would appreciate it if someone would review it.

It's not made any easier by:
1) Catering for systems without makeinfo - I would suggest dropping this.
2) All the nested phony info rules.

Other possible improvements:
1) build-aux/make-info-dir to parse .org sources (for texinfo_dir_category)
in preference to .texi ones if the former exist.
2) Replace dependency on info files with one on generated texi files,
where it makes sense.

(Personally I find this quite a PITA, both from the point of view of
Makefile rules, and from now needing to know two markup syntaxes to
maintain Emacs's manuals. End of complaint.)