[staging] pkgsMusl.which: fix build

[yuyuyureka]

Fixes: #478542

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[mdaniels5757]

Upstream declined to apply this patch for a specific reason, see https://lists.gnu.org/archive/html/which-bugs/2025-04/msg00003.html.

This needs to be gated behind lib.optionals stdenv.hostPlatform.isMusl.

For the reason I gave in #473403 (comment), this is actually likely fine

[mdaniels5757]

I think this is likely fine, but am not quite confident enough to merge it myself. Built {pkgsMusl,}which on aarch64-linux.

[wolfgangwalther]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 477577 --package pkgsMusl.which --package which
Commit: 45b1bbef701502130d1e97ecd681dd0909365bfe

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x86_64-linux

✅ 2 packages built:

• pkgsMusl.which
• which

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aarch64-linux

✅ 2 packages built:

• pkgsMusl.which
• which

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still double-checking the build for aarch64-darwin.

[wolfgangwalther]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 477577 --package which
Commit: 45b1bbef701502130d1e97ecd681dd0909365bfe

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aarch64-darwin

✅ 1 package built:

• which

[yuyuyureka]

After more research, I found the comment originates from an old version of getopt.h from gnulib. The comment was removed in gnulib with commit coreutils/gnulib@1c64ecc#diff-acd0cc21a35657b5071913f8865087c0c0598e701419d32036c7dcfbf22e29bdL148 from 2004...