Fix compilation warnings triggered by Python's pyerrors.h

[Tom Tromey]

>>>>> "Eli" == Eli Zaretskii <eliz@gnu.org> writes:

Eli> Suggested work-around: Define HAVE_SNPRINTF in defs.h:

Why defs.h and not python-internal.h?
I think that, if it works, it is better to try to contain the weirdness
to the Python code.
If it doesn't work, then the patch is ok; and for 7.6 as well.

thanks,
Tom