Apply Safe Haskell flag for newer GHCs only.

Apparently with GHC 7.2.2 and Cabal 1.16, the Data.Set module is not marked as being Safe.

src/Compiler/Hoopl/Passes/Dominator.hs

 {-# LANGUAGE CPP, GADTs #-}
 {-# OPTIONS_GHC -Wall -fno-warn-name-shadowing #-}
-#if __GLASGOW_HASKELL__ >= 701
+#if __GLASGOW_HASKELL__ >= 723
 {-# LANGUAGE Safe #-}
 #endif