fixup #if 0 stubs to use #ifndef mingw32_HOST_OS

That's needed in files used to build the configure program.
For the other files, I'm keeping my __WINDOWS__ define, as I find that much easier to type.
I may search and replace it to use the mingw32_HOST_OS thing later.

Build/DesktopFile.hs

@@ -22,7 +22,7 @@ import Assistant.Install.Menu
 import Control.Applicative
 import System.Directory
 import System.Environment
-#if 0
+#ifndef mingw32_HOST_OS
 import System.Posix.User
 import System.Posix.Files
 #endif
@@ -30,7 +30,7 @@ import System.FilePath
 import Data.Maybe
 
 systemwideInstall :: IO Bool
-#if 0
+#ifndef mingw32_HOST_OS 
 systemwideInstall = isroot <||> destdirset
   where
 	isroot = do