Housekeeping for GHC 8

- Remove some CPP needed only because of GHC 7.4 (which is not supported
  now, yey)
- Move CPP for GHC 8 to Gap module

test/MonadSpec.hs

@@ -23,16 +23,18 @@ spec = do
 
           mv_ex :: MVar (Either SomeException ())
               <- newEmptyMVar
-          mv_startup_barrier :: MVar () <- newEmptyMVar
+          mv_startup_barrier :: MVar ()
+              <- newEmptyMVar
 
           _t1 <- forkOS $ do
-                 putMVar mv_startup_barrier ()
                  -- wait (inside GhcModT) for t2 to receive the exception
-                 _ <- runD $ liftIO $ readMVar mv_ex
+                 _ <- runD $ liftIO $ do
+                            putMVar mv_startup_barrier ()
+                            readMVar mv_ex
                  return ()
 
           _t2 <- forkOS $ do
-                 readMVar mv_startup_barrier -- wait for t1 to start up
+                 readMVar mv_startup_barrier -- wait for t1 to be in GhcModT
                  res <- try $ runD $ return ()
                  res' <- evaluate res
                  putMVar mv_ex res'