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.

CmdLine.hs

@@ -17,7 +17,7 @@ import qualified Control.Exception as E
 import qualified Data.Map as M
 import Control.Exception (throw)
 import System.Console.GetOpt
-#if 0
+#ifndef mingw32_HOST_OS
 import System.Posix.Signals
 #endif