Windows: generate the .exe extension even if the executable name contains a dot.

Fixes #711. (Hopefully)

Cabal/Distribution/Simple/GHC.hs

@@ -814,7 +814,9 @@ buildExe verbosity _pkg_descr lbi
 
   -- exeNameReal, the name that GHC really uses (with .exe on Windows)
   let exeNameReal = exeName' <.>
-                    (if null $ takeExtension exeName' then exeExtension else "")
+                    (if takeExtension exeName' /= exeExtension
+                       then exeExtension
+                       else "")
 
   let targetDir = pref </> exeName'
   let exeDir    = targetDir </> (exeName' ++ "-tmp")