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.

Utility/CopyFile.hs

@@ -36,7 +36,7 @@ copyFileExternal src dest = do
 {- Create a hard link if the filesystem allows it, and fall back to copying
  - the file. -}
 createLinkOrCopy :: FilePath -> FilePath -> IO Bool
-#ifndef __WINDOWS__
+#ifndef mingw32_HOST_OS
 createLinkOrCopy src dest = go `catchIO` const fallback
   where
   	go = do