[etc,config,core] remove leftover Makefiles or add deprecation warnings

[ferdymercury]

Split-off #21820 as requested by @pcanal and @linev

[github-actions]

Test Results

    21 files      21 suites   3d 7h 11m 34s ⏱️
 3 868 tests  3 868 ✅ 0 💤 0 ❌
73 811 runs  73 811 ✅ 0 💤 0 ❌

Results for commit 2f3dea8.

♻️ This comment has been updated with latest results.

[linev]

Makefile.arch, Makefile.config, Makefile.comp are files which were used by external projects for compilation.
I would not delete them now.

We should first fully free ROOT itself from all Makefiles

[ferdymercury]

We should first fully free ROOT itself from all Makefiles

After #21890 goes in, we're almost there.

Then, there are only 21 Makefiles left, all within roottest:

scripts/Makefile
scripts/Makefile.template
Makefile
root/io/evolution/versions/lhcb/Makefile
root/io/withoutDictionaries/Makefile
root/Makefile
root/rint/Makefile
root/treeformula/Makefile
root/meta/Makefile
root/meta/autoloading/headerParsingOnDemand/Makefile
root/meta/autoloading/classInNamespace/Makefile
root/meta/ROOT-7181/Makefile
root/meta/genreflex/ROOT-5594/Makefile
root/meta/genreflex/ROOT-5626/Makefile
root/meta/iotypeioname/Makefile
root/meta/rootcling/Makefile
root/meta/rootcling/ROOT-7695/Makefile
root/tree/Makefile
root/tree/readfile/Makefile
root/tree/cache/Makefile
root/tree/addresses/Makefile

They don't seem long, so doable, but I am not well acquainted with roottest infrastructure, so I won't be able to convert those myself.

[pcanal]

Makefile.arch, Makefile.config, Makefile.comp are files which were used by external projects for compilation.

It probably make sense to replace them (for a release or two) by almost empty files with a error message when used.

[ferdymercury]

Makefile.arch, Makefile.config, Makefile.comp are files which were used by external projects for compilation.

It probably make sense to replace them (for a release or two) by almost empty files with a error message when used.

I added a deprecation warning and we can then remove them after November's 6.42 ?

[guitargeek]

@ferdymercury, this needs a rebase.

Also, our users prefer clear removal schedules for planning. If we're not in a rush to remove this, what about "will be removed in ROOT 7?"

[guitargeek]

LGTM! Thanks.

Review addressed by not removing the Makefiles. Instead, they are deprecated with a long deprecation Window, and only removed in ROOT 7.