WAF: externally setting HAVE_IPV6 config variable

[Uri Simchoni]

Hi,

When cross-building Samba 4.2.0 for a custom embedded device, I've
encountered the following issue:
- The configure script sets HAVE_IPV6
- The device's kernel has IPv6 disabled
- Consequently smbd fails to listen on 0.0.0.0 (b/c it tries a PF_INET6 socket)

The root cause is probably a malformed test - even in native build,
the way to check whether you have IPv6 is to run something against the
kernel, one cannot determine that by compiling alone. I can try to
find a proper test elsewhere and incorporate it as a patch.
However, for the time being my aim is to build without any patches.
I'm curious whether it's possible to force HAVE_IPV6 to false by means
of a command line option (as with autoconf ac_cv_xxx)

Thanks,
Uri.