System c++ stdlib is not detected by default

Prerequisites

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

LEAN_CXX_STDLIB is hardcoded based on the target instead of detected from the default system c++ stdlib.

Context

I have confirmed that applying a patch to the system package works, but it would be better if it was detected correctly upstream with no local/distro patch required.

Steps to Reproduce

1. Compile lean without passing the correct -DLEAN_CXX_STDLIB manually.
2. lake init
3. lake build

Expected behavior: Build completed successfully (8 jobs).

Actual behavior: trace log prints a cc command with the wrong c++ stdlib, and fails to link with many undefined references.

Versions

Lean (version 4.23.0, x86_64-pc-linux-gnu, Release)
6.18.9-gentoo

Additional Information

$ eselect profile show
Current /etc/portage/make.profile symlink:
  default/linux/amd64/23.0/llvm/systemd

Impact

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Start by locating where LEAN_CXX_STDLIB is selected from the target during the Lean build configuration. Reproduce with the reported Gentoo system using lake init and lake build, then inspect the traced cc command. Done means the default system C++ standard library is detected and the build completes without undefined-reference linker errors.