Why do we define HAVE_GSSAPI_EXT_H?

Hi,

Since

commit f7431bca8b0138bdbce7025871560d39119565a0
Author: Stephen Frost <sfrost(at)snowman(dot)net>
Date: 2023-04-13 08:55:13 -0400

Explicitly require MIT Kerberos for GSSAPI

WHen building with GSSAPI support, explicitly require MIT Kerberos and
check for gssapi_ext.h in configure.ac and meson.build. Also add
documentation explicitly stating that we now require MIT Kerberos when
building with GSSAPI support.

configure/meson define HAVE_GSSAPI_EXT_H / HAVE_GSSAPI_GSSAPI_EXT_H - but
afaict we don't use those anywhere?

It makes sense to test for the presence of gssapi_ext.h, but given that we
require it to be present, I think it's not worth emitting HAVE_GSSAPI_EXT_H
etc.

As f7431bca8b is in 16, it seems best to just change this in 18.

While looking at this I also found an argument omission present in the commit
adding meson support. I plan to fix that with the attached commit.

Greetings,

Andres Freund