Fix INITIAL_MARK_STACK_SIZE override in boehm-gc

[edolstra]

Motivation

Nixpkgs now uses structured attrs (i.e. env.<VAR>), so this override wasn't working anymore. It makes a huge performance impact once GC kicks in (e.g. it speeds up a 12-core nix search nixpkgs from 10.5 to 6.5 seconds).

Context

Summary by CodeRabbit

• Chores
  • Updated build configuration handling to combine compiler flags more consistently.
  • Preserved existing compiler-specific options while improving how environment settings are applied.

[coderabbitai]

📝 Walkthrough

Walkthrough

The boehmgc package override in packaging/dependencies.nix is modified so NIX_CFLAGS_COMPILE is set through a merged env attribute (preserving any existing attrs.env) rather than direct assignment, and the flags value is now produced via toString on the flag list instead of a raw list.

Changes

boehmgc override update

Layer / File(s)	Summary
Env-based NIX_CFLAGS_COMPILE assignment packaging/dependencies.nix	Changes NIX_CFLAGS_COMPILE assignment to merge into attrs.env and stringify the flags list via toString, retaining conditional clang/static/LLVM+libcxx flag logic.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: fixing the boehm-gc override after Nixpkgs switched to structured env attributes.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-initial-mark-stack-size

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[github-actions]

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