get rid of __WINDOWS__, use mingw32_HOST_OS

The latter is harder for me to remember, but avoids build failures in code
used by the configure program.

Git/CurrentRepo.hs

@@ -41,7 +41,7 @@ get = do
 			return $ addworktree wt r
   where
 	pathenv s = do
-#ifndef __WINDOWS__
+#ifndef mingw32_HOST_OS
 		v <- getEnv s
 		case v of
 			Just d -> do