fixes/cleanup for use-system-unwind make flags

[benlorenz]

Assuming non-windows and libunwind not disabled:

The flag -DLLVMLIBUNWIND is currently set on macos only for USE_SYSTEM_UNWIND=0 which seems wrong to me and causes build issues for macos on Yggdrasil in combination with the recent #55049 which should only affect gnu libunwind (error: call to undeclared function 'unw_ensure_tls').
This flag is now set independently of the system-libunwind flag (on Darwin and OpenBSD as before).

LIBUNWIND=-lunwind is set for USE_SYSTEM_UNWIND=0 || USE_SYSTEM_UNWIND=1 && OS != Darwin.
I don't think the check for Darwin make sense and might be a leftover from using osxunwind a (long) while ago.
Changed that to always set -lunwind if enabled.

x-ref: JuliaPackaging/Yggdrasil#9331

[fingolfin]

It says "only for linux" here, but wouldn't this also be applied for OpenBSD and FreeBSD if USE_SYSTEM_LIBUNWIND=1 holds? Or maybe that can't happen? (Of course this is a question about the existing code, not something specific to this PR, so it is irrelevant for the acceptance of this PR)

[fingolfin]

Looking at the new logic, wouldn't the following happen on OpenBSD: if USE_SYSTEM_LIBUNWIND=1 holds then both -DSYSTEM_LIBUNWIND and -DLLVMLIBUNWIND are added to JCPPFLAGS.

But perhaps this fine and/or this combo can not happen?

[gbaraldi]

Yeah seems ok to me. The system libunwind in macos is LLVM's anyway

[benlorenz]

I think this one ifneq ($(OS),Darwin) might have been missed in the OpenBSD refactor and should have included OpenBSD as well. So I did another simplification:

• On OpenBSD and Darwin: always set -DLLVMLIBUNWIND since these are the platforms where the llvm libunwind is used.
• Else, on Linux and FreeBSD: set -DSYSTEM_LIBUNWIND if USE_SYSTEM_LIBUNWIND is set to disable some extra gnu libunwind features which are only available in a custom patched gnu libunwind.
• Windows or disabled libunwind: no change.

Looking at the new logic, wouldn't the following happen on OpenBSD: if USE_SYSTEM_LIBUNWIND=1 holds then both -DSYSTEM_LIBUNWIND and -DLLVMLIBUNWIND are added to JCPPFLAGS.

But perhaps this fine and/or this combo can not happen?

This cannot happen anymore now (but I think would be fine anyway).

[fingolfin]

Looks good to me, thanks!