This reverts commit 178c4dfbf4eb5ad7c5aef4838f190f717f97766f.

Instead of creating an MR, I forgot I was on master...

GIT_SILENT

For example, if you have:
//This is a comment

now clang-format won't change it to:
// This is a comment

We're mainly interested in formatting the code, not the comments, so that
cuts down on some noise when running clang-format.

CCBUG: 456819

Only install pre-commit hooks if KDE_CONFIGURE_GIT_PRE_COMMIT_HOOK
is called frm CMAKE_CURRENT_PROEJECT, eg. don't install pre-commit
hooks if ECM is inside a submodule or fetched-content.

The main target consumers of this macro currently (should) want to be
informed about new deprecations as early as possible. As do the authors
of the warnings.

So instead let's make no warnings an opt-in. Not documenting the
now deprecated flag in the docs, given no wide-spread use yet, just
supporting still in the code with a note to users.

This should catch undefined preprocessor expressions, i.e. instead of
showing a warning if FOO_BAR_H is undefined:
 #if FOO_BAR_H

make the build fai lwith an error instead.

This combined with using #cmakedefine01 (instead of #cmakedefine), could
catch things like:
https://invent.kde.org/pim/kdepim-runtime/-/commit/05a0e5bbfbb06ed6aa88640f25f563cf7e5b1330

Should help readers being aware of pitfalls

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