Allow abseil-cpp to cross compile to windows

[LoganEvans]

Description of changes

This adds a build input to include windows pthreads when the host platform is windows.

There are two basic ways to allow cross compiling to windows -- this method allows the abseil code to run #include <pthread.h>.

An alternative (not used here) approach would be to rely on the _WIN32 precompiler directive. The abseil code is designed to not import pthread.h when that symbol is defined. However, this definition is not currently preserved from the mingw compiler. It's not clear what fix would enable this approach.

Things done

These builds now work:

$ nix build .#legacyPackages.x86_64-linux.pkgsCross.mingwW64.abseil-cpp_202301
$ nix build .#legacyPackages.x86_64-linux.pkgsCross.mingwW64.abseil-cpp_202401
$ nix build .#legacyPackages.x86_64-linux.pkgsCross.mingwW64.abseil-cpp_202407

Coincidentally, this also enables:

$ nix build -L .#legacyPackages.x86_64-linux.pkgsCross.mingwW64.protobuf

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[LoganEvans]

@Ericson2314 For pkgsCross.mingw32, it seemed natural to want to use windows.pthreads, but that doesn't work. Is there a major difference between windows.mingw_x64_pthreads and windows.pthreads?