zlib: on mingw, add another DLL link

Also clean up the expression a little.
It fixes at least libpng's DLL.

pkgs/development/libraries/zlib/default.nix

@@ -52,14 +52,21 @@ stdenv.mkDerivation rec {
 
   crossAttrs = {
     dontStrip = static;
+    dontSetConfigureCross = true;
   } // stdenv.lib.optionalAttrs (stdenv.cross.libc == "msvcrt") {
-    configurePhase=''
-      installFlags="BINARY_PATH=$out/bin INCLUDE_PATH=$out/include LIBRARY_PATH=$out/lib"
-    '';
+    installFlags = [
+      "BINARY_PATH=$(out)/bin"
+      "INCLUDE_PATH=$(dev)/include"
+      "LIBRARY_PATH=$(out)/lib"
+    ];
     makeFlags = [
       "-f" "win32/Makefile.gcc"
       "PREFIX=${stdenv.cross.config}-"
-    ] ++ (if static then [] else [ "SHARED_MODE=1" ]);
+    ] ++ stdenv.lib.optional (!static) "SHARED_MODE=1";
+
+    # Non-typical naming confuses libtool which then refuses to use zlib's DLL
+    # in some cases, e.g. when compiling libpng.
+    postInstall = postInstall + "ln -s zlib1.dll $out/bin/libz.dll";
   } // stdenv.lib.optionalAttrs (stdenv.cross.libc == "libSystem") {
     makeFlags = [ "RANLIB=${stdenv.cross.config}-ranlib" ];
   };