python-gyp: fix darwin build without xcode

[LnL7]

Motivation for this change

This should make http-parser and all of it's dependencies build on hydra.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • macOS
  • No Changes
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@LnL7, thanks for your PR! By analyzing the history of the files in this pull request, we identified @aszlig and @FRidh to be potential reviewers.

[FRidh]

What is the origin of this patch? Did you write this yourself, or does it come from elsewhere?

[LnL7]

It's handwritten based on the previous patch, but it looks like this still needs the CLT 😞

[acowley]

@LnL What happened when you tried to use xcbuild? You said it didn't like an empty path at some point, is that right?

[LnL7]

The MacOSX sdk, didn't really look any further since I thought the patch worked.

[FRidh]

Is this ready or work in progress?

[LnL7]

I guess we could merge this in, this won't work on hydra yet but you don't need a full xcode install anymore.

[retrry]

@LnL7 could you check my branch here: https://github.com/retrry/nixpkgs/tree/fix-gyp-darwin? It has one commit https://github.com/retrry/nixpkgs/commit/cd5955db68615f48e3b1feb6fa59f650ff574bcc. With it everything compiles on my machine, which doesn't have xcode installed. Maybe add that commit to your pull request and that would be enough?

[LnL7]

@retrry Yes, that looks like the missing part!

It's actually the only change that seems to be required.

[retrry]

@FRidh @LnL7 so can we get this merged? :) I would really like to get rust stuff working on Darwin again.

[FRidh]

@retrry that's up to @LnL7

[LnL7]

I noticed some things that use gyp still fail, I want to compare if my changes help in those cases or if it's unrelated.

[acowley]

Yay, this fixed cargo as we had hoped! Thanks, @LnL7!