add nix flake

[caniko]

Summary

This PR adds a comprehensive Nix flake that enables building the OSXCross macOS cross-compilation toolchain in a reproducible, declarative way. The flake provides first-class integration with the Nix ecosystem including Rust cross-compilation helpers for use with rust-overlay and crane.

Alternatively, I can also maintain the flake downstream if you don't want to support nix.

Features

Core Toolchain

• Complete toolchain build - Builds cctools-port, ld64, TAPI library, XAR, and compiler wrappers
• SDK auto-detection - Automatically detects SDK version from tarball filename
• Multi-architecture support - arm64, arm64e, x86_64, x86_64h, i386 (SDK-dependent)
• Configurable options - Deployment target, architecture selection, LTO support

Rust Integration

• rust-overlay compatible - Helpers for setting up Rust cross-compilation
• crane integration - mkCrossBuilder wraps crane for easy cross-builds
• Universal binary support - mkUniversalBinary creates fat binaries from arm64 + x86_64 builds
• Environment helpers - cargoEnvFor, mkCargoConfig, mkDevShellHook

Build Quality

• Minimal shell scripting - Logic implemented in native Nix where possible
• Pure data structures - Symlink definitions as attrsets, not shell commands
• No pkg-config shadowing - Unprefixed pkg-config intentionally omitted to avoid breaking native Linux builds

Usage

{
  inputs.osxcross.url = "github:tpoechtrager/osxcross";

  outputs = { osxcross, ... }: {
    packages.x86_64-linux.myApp = let
      toolchain = osxcross.lib.x86_64-linux.mkOsxcross {
        sdkPath = ./MacOSX14.5.sdk.tar.xz;
      };
    in /* use toolchain */;
  };
}

Files Added

File	Description
flake.nix	Main flake with mkOsxcross, mkRustHelpers, overlay
nix/lib.nix	SDK version mapping, architecture validation, symlink generation
nix/osxcross.nix	Main derivation combining all components
nix/cctools-port.nix	Apple cctools and ld64 linker
nix/wrapper.nix	Compiler wrapper build
nix/sdk.nix	SDK extraction from tarballs
nix/xar.nix	XAR archive tool
nix/apple-libtapi.nix	Apple TAPI library for .tbd files
nix/rust.nix	Rust cross-compilation helpers
nix/README.md	Comprehensive documentation

Testing

# Check flake evaluation
nix flake check --no-build

# Build standalone packages
nix build .#xar
nix build .#apple-libtapi

# Build full toolchain (requires SDK)
nix build --impure --expr '
  (builtins.getFlake (toString ./.)).lib.x86_64-linux.mkOsxcross {
    sdkPath = /path/to/MacOSX14.5.sdk.tar.xz;
  }
'

Commit signature verification

Will sign the commit later.