windows.mingw_w64_headers: fix building against msvcrt

Previously we relied on the default CRT selected by upstream. This
broke builds against MSVCRT starting with 12.0, as the default was
changed to UCRT

We can instead manually configure the CRT based on our `stdenv` to
ensure there is no discrepancy between the CRT we want to use and that
being targeted by the build

pkgs/os-specific/windows/mingw-w64/headers.nix

@@ -2,6 +2,7 @@
   lib,
   stdenvNoCC,
   fetchurl,
+  crt ? stdenvNoCC.hostPlatform.libc,
 }:
 
 stdenvNoCC.mkDerivation (finalAttrs: {
@@ -13,6 +14,10 @@ stdenvNoCC.mkDerivation (finalAttrs: {
     hash = "sha256-zEGJiqxLbo3Vz/1zMbnZUVuRLfRCCjphK16ilVu+7S8=";
   };
 
+  configureFlags = [
+    (lib.withFeatureAs true "default-msvcrt" crt)
+  ];
+
   preConfigure = ''
     cd mingw-w64-headers
   '';