[Axiom-developer] Build machinery -- support for out of source build

[daly]

> You are wrong. A dependency tracking system needs to be part of the
> system, not just at the Make level, but also the system
> level. Otherwise you get into obscure contorsions as exemplified by
> the build of depsys.

Depsys exists for a reason. It handles things like macros in lisp
and it removes things that are only needed at compile time from the
final running image. If you miss the depsys dependencies you'll find
that you get "clean" lisp compiles but the generated code is wrong
due to missing macro expansions. This will show up as runtime bugs
much later in the process, perhaps during algebra computations.

Make and lisp have slightly different ideas about what it means to
depend on something. If you actually list all of the dependencies at
the Make level you'll find that you do a lot of unnecessary work.
This only shows up when you begin to change a single file and do
Make again. In theory Make should do the minimum amount of required
work given all the dependencies. In a lisp based system this is very
hard to do.

A simple example is that the algebra code is built in layers. Each
layer is built depending on the layers beneath it. Originally the
algebra Makefile specified this dependency, which worked fine during
the build. Yet if you change a single algebra file and do a second
Make you can end up rebuilding a lot of the algebra needlessly.

The current system dependencies were determined by doing a system
build and then doing a modification to each kind of file, followed
by a Make. In theory Make should only do a small amount of work to
include only changes required. It took a fair amount of testing to
determine the compromises between what Make needs and what lisp needs.

Getting this right is important and requires a lot of testing.

Tim