Add volatile

[nordlander]

Add C keyword volatile to C variable declarations declared outside an exception handler but referenced inside it. Example:

def f():
   res = 0
   try:
       res = 1
       raise ValueError
   except ValueError:
       return res         # should return 1

Here the generated C code must prepend the declaration of res with volatile, in order to be insensitive to any representation tricks the C compiler might perform on the code inside the try branch.