[Cython] adding more constification

[Lisandro Dalcin]

I'm working on a patch adding 'const' to 'char *' arguments, and a few
explicit (char *) casts.

The final idea is that for Py >= 2.5, we can then pass -Wwrite-strings
to GCC and no warnings should appear. Moreover, perhaps we can make
'runtest.py' a bit smarter from pushing the -Wwrite-strings flags for
Py >=2.5 and GGC.

I did not take the Py2/3/2.4 case just because that would require the
generated C code to have many explicit, ugly (char *) cast everywhere.
But if this is not a problem, it could be done in the near future.

Any objections?


-- 
Lisandro Dalcín
---------------
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
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