Upstream `gcc-13` merged `mcfgthreads` support with a caveat: it's
headers interface is not compatible with the patch `nixpkgs` was
carrying in `gcc-12` and before.
To keep both new (`gcc13`) and old (`_pre_gcc13`) version I held back
previous `windows.mcfgthreads` attribute as
`windows.mcfgthreads_pre_gcc_13`. It is used for `gcc` before 13.
The change fixes the build of `pkgsCross.mingwW64.stdenv` itself and
example program:
$ nix build --impure --expr 'with import ./. {}; pkgsCross.mingwW64.re2c.override { stdenv = pkgsCross.mingwW64.gcc11Stdenv; }'
c6132dcf23