gfortran: fix build for mingwW64

[amesgen]

Description of changes

This fixes the following compilation error, e.g. observable when building pkgsCross.mingwW64.lapack-reference:

../../../gcc-12.2.0/libgfortran/io/async.h:354:3: error: unknown type name 'pthread_t'
  354 |   pthread_t thread;
      |   ^~~~~~~~~

The fix is to replace pthread_t with __gthread_t (which is what pthread_t is defined as in other places), though I am not sure why this is necessary for mingwW64 cross compilation.

I included the test for GCC 11 and 12; I guess that it works for older versions too, but I haven't tested it.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 23.05 Release Notes (or backporting 22.11 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.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/2152

[amesgen]

This seems to have been fixed by #268097 as a side effect, closing.