[Cython] __Pyx_FinishRefcountContext() compiler warnings

[Jason Evans]

The following line in ModuleNode.py causes hundreds of compiler warnings 
about statements with no effect when I compile Crux:

	#define __Pyx_FinishRefcountContext() 0

IMO, the 0 should simply be removed, but this would also be okay if you 
want a non-empty definition:

	#define __Pyx_FinishRefcountContext() do {} while (0)

Jason