gcc: Build MinGW stage two with threading library

Currently this is set up to be mcfgthreads, but it could be something
else instead.

pkgs/development/compilers/gcc/4.8/default.nix

@@ -23,6 +23,7 @@
 , enablePlugin ? stdenv.hostPlatform == stdenv.buildPlatform # Whether to support user-supplied plug-ins
 , name ? "gcc"
 , libcCross ? null
+, threadsCross ? null # for MinGW
 , crossStageStatic ? false
 , # Strip kills static libs of other archs (hence no cross)
   stripped ? stdenv.hostPlatform == stdenv.buildPlatform
@@ -47,10 +48,14 @@ assert stdenv.hostPlatform.isDarwin -> gnused != null;
 # The go frontend is written in c++
 assert langGo -> langCC;
 
+# threadsCross is just for MinGW
+assert threadsCross != null -> stdenv.targetPlatform.isWindows;
+
 with stdenv.lib;
 with builtins;
 
-let version = "4.8.5";
+let majorVersion = "4";
+    version = "${majorVersion}.8.5";
 
     inherit (stdenv) buildPlatform hostPlatform targetPlatform;
 
@@ -171,6 +176,8 @@ stdenv.mkDerivation ({
     ++ (optional hostPlatform.isDarwin gnused)
     ;
 
+  depsTargetTarget = optional (!crossStageStatic && threadsCross != null) threadsCross;
+
   preConfigure = import ../common/pre-configure.nix {
     inherit (stdenv) lib;
     inherit version hostPlatform langJava langGo;
@@ -253,7 +260,7 @@ stdenv.mkDerivation ({
 
   inherit
     (import ../common/extra-target-flags.nix {
-      inherit stdenv crossStageStatic libcCross;
+      inherit stdenv crossStageStatic libcCross threadsCross;
     })
     EXTRA_TARGET_FLAGS
     EXTRA_TARGET_LDFLAGS