Merge Makefiles

[kinke]

Depends on dlang/dmd#15894 + dlang/dmd#15920 (pushed as same-named DMD branch makefiles for CI), then seems to work already with these pretty minimal posix.mak adaptations (see first commit - review commit-by-commit). :)

For CI, this makes use of the special feature that branch CI pipelines (for branches pushed to the dlang repo) try to clone a same-named branch on the other dlang repo. So the relevant pipelines to look at are the branch pipelines, not the PR pipelines. For GitHub Actions, the branch pipeline jobs are displayed as (push) (vs. (pull_request)) here in the GUI; for Azure CI, it needs to be looked up manually.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8862"

[kinke]

Wrt. zlib (the only C parts of Phobos), the situation was a bit wild:

• posix.mak compiled the 15 .c files directly, using the C compiler.
• win*.mak had misleading rules for compiling .c files with DMD via importC, but was actually using etc/c/zlib/win{32,64}.mak, which was using the C compiler.

With the generic Makefile now, the files are compiled directly in there, using the C compiler for now. The zlib Makefiles are unused, and after seeing that they seem to be additions of ours (incl. {linux,osx}.mak) , not part of upstream https://github.com/madler/zlib/tree/v1.2.12, I've killed them with fire.

[thewilsonator]

Dependant PR merged.

[kinke]

Thanks! - Landing this probably breaks Windows CI for DMD master, requiring dlang/dmd#15920 to land in tandem.