pythonPackages.lxml: fix build on darwin

[bobrik]

Motivation for this change

Update in #107408 broke it for Darwin.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS (aarch64)
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[vcunat]

I believe this needs to be rebased on staging-next (i.e. some previous commits removed) and retargeted against staging-next.

[SuperSandro2000]

I believe this needs to be rebased on staging-next (i.e. some previous commits removed) and retargeted against staging-next.

git rebase --onto=staging --onto=staging-next --onto=master HEAD~1

[bobrik]

@SuperSandro2000 your command seems to rebase onto master:

$ git rebase --onto=origin/staging --onto=origin/staging-next --onto=origin/master HEAD~1
Current branch ivan/lxml-darwin is up to date.

$ git log -2
commit fd04a5e15401f1ae17791c5e0ac66cc423f0f70f (HEAD -> ivan/lxml-darwin)
Author: Ivan Babrou <github@ivan.computer>
Date:   Wed Mar 17 13:32:48 2021 -0700

    pythonPackages.lxml: fix build on darwin

commit ea68396a78b6143fc97c8319ba29c408a80faf9f (origin/master, origin/HEAD)
Merge: 3643876a3c1 59617f748e5
Author: Sandro <sandro.jaeckel@gmail.com>
Date:   Thu Mar 18 01:41:03 2021 +0100

    Merge pull request #101510 from omasanori/libressl-3.2.2

    libressl_3_2: init at 3.2.5, libressl_3_1: 3.1.4 -> 3.1.5

It may be easier to ask the bot to do the right thing.

[SuperSandro2000]

/rebase staging

[github-actions]

Failed to rebase

[bobrik]

staging-next, not staging?

[SuperSandro2000]

@SuperSandro2000 your command seems to rebase onto master:

🤔 might be. Using the bot is not a bad idea.

[SuperSandro2000]

/rebase staging-next

[github-actions]

Rebased, please reopen the pull request to restart CI

[thefloweringash]

The problem is setupinfo.py setting _ldflags=['-isysroot', ''] and and some layer the empty string is omitted. It determines the location of the sysroot from the output of xcrun --show-sdk-path. This PR fixes it by adding an xcrun in the build environment that provides a valid path instead of the empty string, but it's also possible to patch the _ldflags out of setupinfo.py entirely and avoid the dependency. Either way works, but this requires less patching, so LGTM.