Fixed compilation of IECoreAppleseed on OS X.

Without constructing the pointer separately, GCC 4.2.1 was giving the following error :

contrib/IECoreAppleseed/src/IECoreAppleseed/bindings/ToAppleseedConverterBinding.cpp:59: error: no matching function for call to 'foundation::auto_release_ptr<renderer::Entity>::auto_release_ptr(foundation::auto_release_ptr<renderer::Entity>)'
Fixed linking of IECoreAppleseed module on OS X.
Merge pull request #363 from johnhaddon/appleseedOSXFixes

Appleseed OS X fixes