openconnect: fix missing iconv on Darwin

[sarahec]

Hydra: 326248787

Build fails on Darwin with "__iconv undefined".
Set env.am_cv_func_iconv_works = "yes" so it uses the existing library.

Tracking: #511329

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.

[booxter]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 511586 --package openconnect
Commit: 00bd3ffc1a00d5e476f9a27c584ca50aac1783af

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x86_64-linux

✅ 2 packages built:

• openconnect
• openconnect.dev (openconnect.dev.dev)

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aarch64-linux

✅ 2 packages built:

• openconnect
• openconnect.dev (openconnect.dev.dev)

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aarch64-darwin

✅ 2 packages built:

• openconnect
• openconnect.dev (openconnect.dev.dev)