Kerberos warnings on win32

When building with Kerberos support (or GSSAPI, but not SSPI) on Win32, a
whole bunch of warnings come out due to redefinitions of macros in the
kerberos headers. The reason for this is that Kerberos leaks the
HAVE_<something> macros from autoconf into the header files that are
included by PostgreSQL.

The attached file removes this by undefing the macros before we include the
kerberos files. But this is perhaps just too ugly to deal with and we
should live with the warnings instead?

For MSVC, we can suppress the warnings with a #pragma around the include,
but I don't know if that's possible in mingw/gcc. It'll look something likt
the second patch attached (only did that for one of the places that'd need
it, to show what it looks like)

Thoughts? Worth doing anything about?

//Magnus