Makefile: wire up build option for deprecated features

Makefile: wire up build option for deprecated features

With 57ec9254eb (docs: introduce document to announce breaking changes,
2024-06-14), we have introduced a new document that tracks upcoming
breaking changes in the Git project. In 2454970930 (BreakingChanges:
early adopter option, 2024-10-11) we have amended the document a bit to
mention that any introduced breaking changes must be accompanied by
logic that allows us to enable the breaking change at compile-time.
While we already have two breaking changes lined up, neither of them has
such a switch because they predate those instructions.

Introduce the proposed `WITH_BREAKING_CHANGES` preprocessor macro and
wire it up with both our Makefiles and Meson. This does not yet wire up
the build flag for existing deprecations.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>