boost: Supply some configure flags for mingw.

Otherwise, Boost.Build is trying to compile against pthread and
desperately searches for icu/iconv.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/development/libraries/boost/generic.nix

@@ -76,6 +76,9 @@ let
     "--user-config=user-config.jam"
     "toolset=gcc-cross"
     "--without-python"
+  ] ++ optionals stdenv.isCrossWin [
+    "target-os=windows"
+    "threadapi=win32"
   ];
   crossB2Args = concatStringsSep " " (genericB2Flags ++ crossB2Flags);