mingw-w64: Clean up, especially clarifying staging

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

@@ -0,0 +1,11 @@
+{ fetchurl }:
+
+rec {
+  version = "4.0.6";
+  name = "mingw-w64-${version}";
+
+  src = fetchurl {
+    url = "mirror://sourceforge/mingw-w64/mingw-w64-v${version}.tar.bz2";
+    sha256 = "0p01vm5kx1ixc08402z94g1alip4vx66gjpvyi9maqyqn2a76h0c";
+  };
+}

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

@@ -1,36 +1,6 @@
-{ stdenv, fetchurl, binutils ? null, gccCross ? null
-, onlyHeaders ? false
-, onlyPthreads ? false
-}:
-
-let
-  version = "4.0.6";
-  name = "mingw-w64-${version}";
-in
-stdenv.mkDerivation ({
-  inherit name;
-
-  src = fetchurl {
-    url = "mirror://sourceforge/mingw-w64/mingw-w64-v${version}.tar.bz2";
-    sha256 = "0p01vm5kx1ixc08402z94g1alip4vx66gjpvyi9maqyqn2a76h0c";
-  };
-} //
-(if onlyHeaders then {
-  name = name + "-headers";
-  preConfigure = ''
-    cd mingw-w64-headers
-  '';
-  configureFlags = "--without-crt";
-} else if onlyPthreads then {
-  name = name + "-pthreads";
-  preConfigure = ''
-    cd mingw-w64-libraries/winpthreads
-  '';
-} else {
-  buildInputs = [ gccCross binutils ];
-
-  crossConfig = gccCross.crossConfig;
+{ stdenv, callPackage }:
 
+stdenv.mkDerivation {
+  inherit (callPackage ./common.nix {}) name src;
   dontStrip = true;
-})
-)
+}

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

@@ -0,0 +1,13 @@
+{ stdenvNoCC, callPackage }:
+
+let
+  inherit (callPackage ./common.nix {}) name src;
+
+in stdenvNoCC.mkDerivation {
+  name = name + "-headers";
+  inherit src;
+
+  preConfigure = ''
+    cd mingw-w64-headers
+  '';
+}

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

@@ -0,0 +1,13 @@
+{ stdenvNoCC, callPackage }:
+
+let
+  inherit (callPackage ./common.nix {}) name src;
+
+in stdenvNoCC.mkDerivation {
+  name = name + "-pthreads";
+  inherit src;
+
+  preConfigure = ''
+    cd mingw-w64-libraries/winpthreads
+  '';
+}