iphlpapi: Don't rely on the HAVE_XXX macros having a numeric values.

[Dan Kegel]

Alexandre wrote:
> It's common practice to avoid it, to make things like -Wundef more useful.

Good point.

Reading http://ozlabs.org/~rusty/index.cgi/tech/2008-01-04.html makes
me sad.  -Wundef would be a lot more useful if autoconf defined symbols
to zero instead of undefining them; then we could get rid of all the defined()
calls, and the compiler could tell us when we'd misspelled a HAVE_XXX.
 Oh, well.
- Dan