fix: use libtool instead of ar for static libs on macOS

[kim-em]

This PR fixes a build failure on macOS introduced by #12540. macOS BSD ar does not support the @file response file syntax that #12540 enabled unconditionally. On macOS, when building core (i.e., bootsrap := true), recBuildStatic now uses libtool -static -filelist, which handles long argument lists natively.

Includes a stage0/src/stdlib_flags.h trigger so CI will automatically run update-stage0 after merge.

🤖 Prepared with Claude Code

Implementation adjusted by @tydeu

[tydeu]

If I understand correctly, this is just for core, right? I do not like that this makes the ar generally optional in the toolchain.

Instead, for example, the Lean library in recBuildStatic could set some option for buildStaticLib (e.g., useLibtool or even bootstrap) indicating one should use libtool on macOS when in core -- i.e., self.pkg.bootstrap is true . There is already a special case there for the use of thin in core. Alternatively, it could just inline the buildStaticLib code and pass none for compileStaticLib's AR when bootstrapping.

In essence, I think we should make this as local as possible to the case where things break (i.e., bootstrapping core) and minimally impact the API in doing so.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

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[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase bf4f51e70471a4828855f69967345c27869ad2ad --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-18 02:01:32)