Fixed clang build.

TBB doesn't actually define the task_scheduler_init::automatic symbol, and clang wants to reference it rather than evaluate the constant. I don't quite understant it fully, but there's some detail on a similar problem here :

http://stackoverflow.com/questions/5391973/undefined-reference-to-static-const-int

src/GafferModule/GafferModule.cpp

@@ -178,11 +178,11 @@ BOOST_PYTHON_MODULE( _Gaffer )
 	DependencyNodeClass<SwitchComputeNode>();
 
 	object tsi = class_<TaskSchedulerInitWrapper, boost::noncopyable>( "_tbb_task_scheduler_init", no_init )
-		.def( init<int>( arg( "max_threads" ) = tbb::task_scheduler_init::automatic ) )
+		.def( init<int>( arg( "max_threads" ) = int( tbb::task_scheduler_init::automatic ) ) )
 		.def( "__enter__", &TaskSchedulerInitWrapper::enter, return_self<>() )
 		.def( "__exit__", &TaskSchedulerInitWrapper::exit )
 	;
-	tsi.attr( "automatic" ) = tbb::task_scheduler_init::automatic;
+	tsi.attr( "automatic" ) = int( tbb::task_scheduler_init::automatic );
 
 	object behavioursModule( borrowed( PyImport_AddModule( "Gaffer.Behaviours" ) ) );
 	scope().attr( "Behaviours" ) = behavioursModule;