Remove extra-lib-dirs from OSX build

This is the *right* way to solve the problem, except that it has the unfortunate side-effect of requiring at least ghc 7.2.1.

As you can see in the GHC release notes: http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/release-7-2-1.html

That version of ghc was released August 9th of 2011.  The current and previous releases of the Haskell Platform are both sufficiently new to support this feature: http://www.haskell.org/platform/changelog.html

I think this is a good change from a maintainability point of view and I think it's safe now that the versions of ghc that support this have been out for about 2 years, including a few HP releases.