treewide: undo changes causing rebuilds

pkgs/development/compilers/gcc/patches/default.nix

@@ -224,7 +224,7 @@ in
   .${majorVersion} or [ ]
 )
 
-++ optional (targetPlatform.isWindows || targetPlatform.isCygwin) (fetchpatch {
+++ optional targetPlatform.isCygwin (fetchpatch {
   name = "libstdc-fix-compilation-in-freestanding-win32.patch";
   url = "https://inbox.sourceware.org/gcc-patches/20250922182808.2599390-2-corngood@gmail.com/raw";
   hash = "sha256-+EYW9lG8CviVX7RyNHp+iX+8BRHUjt5b07k940khbbY=";

pkgs/stdenv/generic/make-derivation.nix

@@ -706,14 +706,19 @@ let
               __propagatedImpureHostDeps = computedPropagatedImpureHostDeps ++ __propagatedImpureHostDeps;
             }
           )
-          // optionalAttrs (isWindows || isCygwin) {
-            allowedImpureDLLs =
-              allowedImpureDLLs
-              ++ lib.optionals isCygwin [
-                "KERNEL32.dll"
-                "cygwin1.dll"
-              ];
-          }
+          // optionalAttrs (isWindows || isCygwin) (
+            let
+              dlls =
+                allowedImpureDLLs
+                ++ lib.optionals isCygwin [
+                  "KERNEL32.dll"
+                  "cygwin1.dll"
+                ];
+            in
+            {
+              allowedImpureDLLs = if dlls != [ ] then dlls else null;
+            }
+          )
           // (
             if !__structuredAttrs then
               makeOutputChecks attrs