bug#48117: 28.0.50; Update of loaddefs.el during normal build is unreliable

[Glenn Morris]

This issue has been present forever.
There are some comments in the Makefiles about it.
There are several issues, eg:

1) autoload generation is slow.

2) the dependencies of the loaddefs files are unknown to make,
and are basically "all lisp files". (You can't even say "just those
files with autoload statements", because removing a previously existing
autoload statement changes the output.)

3) Traditionally, re-making loaddefs files could make trivial changes
to the output that weren't important (eg ordering of the "no
autoloads" section, timestamping), but would still trigger re-dumping emacs.
Which could then trigger regeneration of the autoloads, and
re-dumping, etc.  This may be better nowadays, since there is no
longer timestamp information in the loaddefs files (see autoload-timestamps).