build: defer libwhich lookup in symlink_system_library

[fingolfin]

Run libwhich -p inside the symlink recipe shell instead of
using $(shell ...) in a recursively expanded make variable.

This keeps evaluation timing explicit and avoids unpredictable
expansion behavior from recursive $(shell ...).

The lookup still happens only when the symlink target is built,
so default builds and clean targets do not trigger unnecessary
probes.

This fixes in particular annoying segfaults in libwhich on macOS when
it tries to inspect libcrypto or libssl. Since USE_SYSTEM_OPENSSL=0
by default, the failed lookups don't matter but they look scary and
if CrashReporter is active, can be also rather annoying.

Resolves #57670

Co-authored-by: Codex codex@openai.com

[Keno]

$(shell inside a recursive expansion is an antipattern, because the expansion point becomes unpredictable. My suggestion would be a simply-expanded $(eval inside the target definition. See e.g. https://stackoverflow.com/questions/1909188/define-make-variable-at-rule-execution-time

[Keno]

Actually, I might be wrong about when $(eval runs. In that case these could be made shell variables. My point is to avoid the $(shell) in the recursive expansion.

[Keno]

Never mind, just double checked, $(eval is fine.

[Keno]

Although in this case may need to be $$(eval of course.

[fingolfin]

OK, I (or rather Codex 😂) rewrote it with a different approach which I hope addresses your concern (at least it does seem sensible to me)

[Keno]

Yes, codex did the In that case these could be made shell variables suggestion I made above, which works fine.