[PATCH 3/3] configure: Use 0 instead of NULL because NULL isn't defined.

[Juan Lang]

> Both "0" and "(void *) 0" work here; which to use is a matter of style
> and thus good for a flame war. But i don't care too much to start one ;)

I'll point out that for the case at hand, a configure test, code
readability is probably not a very important goal.  As Ken points out,
the configure check failed before, but for the wrong reason - NULL
wasn't defined.  Now it'll fail for the right reason - the function
doesn't exist on MacOS X.  The compiler warning during the configure
check is only apparent in the configure log, which is probably why the
reason for this failing was missed for so long.

--Juan