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.

Command/Unannex.hs

@@ -66,7 +66,7 @@ cleanupIndirect file key = do
 		)
 	return True
   where
-#ifdef __WINDOWS__
+#ifdef mingw32_HOST_OS
 	goFast = go
 #else
 	goFast = do