Don't require -no_pie on some versions of GHC

[judah]

It looks like -no_pie might not be necessary on ghc>=7.4.1, since the local version of gmp has that problem fixed.

If so, ghc-xcode should not print that flag when using a version of ghc which is recent enough.

[judah]

The GHC ticket for this is: http://hackage.haskell.org/trac/ghc/ticket/5293