Restore compatibility with GHC 7.0

src/runtime/haskell/PGF/Optimize.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE CPP, BangPatterns #-}
 module PGF.Optimize
              ( optimizePGF
              , updateProductionIndices
@@ -11,7 +11,11 @@ import PGF.Macros
 import Data.List (mapAccumL)
 import Data.Array.IArray
 import Data.Array.MArray
+#if MIN_VERSION_base(4,6,0)
 import Data.Array.Unsafe as U(unsafeFreeze)
+#else
+import Data.Array.ST as U(unsafeFreeze)
+#endif
 import Data.Array.ST
 import Data.Array.Unboxed
 import qualified Data.Map as Map