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.

Git/Construct.hs

@@ -23,7 +23,7 @@ module Git.Construct (
 	checkForRepo,
 ) where
 
-#if 0
+#ifndef mingw32_HOST_OS
 import System.Posix.User
 #endif
 import qualified Data.Map as M hiding (map, split)