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.

Creds.hs

@@ -17,7 +17,7 @@ import Types.Remote (RemoteConfig, RemoteConfigKey)
 import Remote.Helper.Encryptable (remoteCipher, embedCreds)
 
 import System.Environment
-#if 0
+#ifndef mingw32_HOST_OS
 import System.Posix.Env (setEnv)
 #endif
 import qualified Data.ByteString.Lazy.Char8 as L