Ups, expat_external.h so this one should be left alone (or thouched upstream). So setting to draft for now,

Not a skip issue IMO because benchmarks reported on the issue + possible discussion on the issue rather than the PR may be worthwile.

Also it's sometimes good to be able to gather similar improvements under an umbrella issue

@chris-eibl could you add a small NEWS entry under the build section? I think it would be good to know that these macros are now enabled when possible (at least for object allocation). Or maybe the formulation would be too obscure? if this is the case, just add skip news and I'll merge.

I'd go with skip news, since according to @zooba #131279 (comment)

[...], since we still aren't technically supporting clang-cl on Windows yet,

clang-cl on Windows is quite niche, and I can't think of anything better than something like

Enable the optimizing macros UNLIKELY and LIKELY for clang-cl on Windows in obmalloc.c.

which is indeed not much help.

I'd happily blurb it like so if this deems reasonable.