disable MonadPlus instance that doesn't compile with 6.6

compiler/utils/IOEnv.hs

@@ -139,9 +139,15 @@ unsafeInterleaveM (IOEnv m) = IOEnv (\ env -> unsafeInterleaveIO (m env))
 
 -- For use if the user has imported Control.Monad.Error from MTL
 -- Requires UndecidableInstances
+#if __GLASGOW_HASKELL__ > 606
+-- for some reason, this doesn't compile with GHC 6.6:
+-- utils/IOEnv.hs:144:33:
+--     No instance for (MonadPlus IO)
+--       arising from use of `mplus' at utils/IOEnv.hs:144:33-67
 instance MonadPlus IO => MonadPlus (IOEnv env) where
     mzero = IOEnv (const mzero)
     m `mplus` n = IOEnv (\env -> unIOEnv m env `mplus` unIOEnv n env)
+#endif
 
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 -- Accessing input/output