gh-115992: Generate build dependencies using gcc facility

[smontanaro]

This PR adds a pair of dependency files, Makefile.deps.Linux and Makefile.deps.Darwin as well as a new regen target, regen-deps for keeping them up-to-date.

This is just a proposal. I expect some discussion and some pushback on the idea. I still think it worth exploring, even in the end it doesn't fly.

• Issue: Build dependencies should be automatically generated #115992

[smontanaro]

Note that I am asymptotically approaching dependency generation Nirvana. The latest update picks up Modules (but also duplicates some Modules which were found in the first go-round). It's an iterative process. I'd rather have something mostly complete out there for people to play with an comment on than aim for perfection on the first try.

[smontanaro]

The latest update to Makefile.deps.Darwin highlights a problem. I updated my Homebrew installation which updated OpenSSL from 3.2.0_1 to 3.2.1. The version number is encoded in the dependencies. Not sure quite what to do there...

[smontanaro]

The latest update to Makefile.deps.Darwin highlights a problem. I updated my Homebrew installation which updated OpenSSL from 3.2.0_1 to 3.2.1. The version number is encoded in the dependencies. Not sure quite what to do there...

Solved by not checking the dependency files into Git. Instead, they are generated as the last step of configure.

[smontanaro]

I gave up trying to shoehorn Module dependencies into this PR. It's just such a nightmare. I'm really surprised we still use Modules/Setup. Does anyone actually build a fully statically linked executable anymore?

(My apologies to the people who have turned that aspect of the build system into a labor of love.)

[smontanaro]

Things went wacky, and I have yet to figure out why. Closing this PR until I get a) more consistent results, and/or b) when I get failures, I can explain them.