windows.mingw_w64_pthreads: put deprecation behind alias

pkgs/os-specific/windows/default.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  config,
   stdenv,
   buildPackages,
   pkgs,
@@ -9,7 +10,9 @@
 }:
 
 lib.makeScope newScope (
-  self: with self; {
+  self:
+  with self;
+  {
     dlfcn = callPackage ./dlfcn { };
 
     mingw_w64 = callPackage ./mingw-w64 {
@@ -31,8 +34,6 @@ lib.makeScope newScope (
 
     mingw_w64_headers = callPackage ./mingw-w64/headers.nix { };
 
-    mingw_w64_pthreads = lib.warn "windows.mingw_w64_pthreads is deprecated, windows.pthreads should be preferred" self.pthreads;
-
     mcfgthreads = callPackage ./mcfgthreads { stdenv = crossThreadsStdenv; };
 
     npiperelay = callPackage ./npiperelay { };
@@ -43,4 +44,7 @@ lib.makeScope newScope (
 
     sdk = callPackage ./msvcSdk { };
   }
+  // lib.optionalAttrs config.allowAliases {
+    mingw_w64_pthreads = lib.warn "windows.mingw_w64_pthreads is deprecated, windows.pthreads should be preferred" self.pthreads;
+  }
 )