build: Depend on guile-ssh 0.12.0

This is a followup to 35f3511167.

* m4/guix.m4 (GUIX_CHECK_GUILE_SSH): Check for userauth-gssapi!
* doc/guix.texi: Document version requirement

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -772,11 +772,11 @@ The following dependencies are optional:
 
 @itemize
 @item
-@c Note: We need at least 0.10.2 for 'channel-send-eof'.
+@c Note: We need at least 0.12.0 for 'userauth-gssapi!'.
 Support for build offloading (@pxref{Daemon Offload Setup}) and
 @command{guix copy} (@pxref{Invoking guix copy}) depends on
 @uref{https://github.com/artyom-poptsov/guile-ssh, Guile-SSH},
-version 0.10.2 or later.
+version 0.12.0 or later.
 
 @item
 When @url{https://www.nongnu.org/lzip/lzlib.html, lzlib} is available, lzlib

m4/guix.m4

@@ -142,13 +142,14 @@ dnl GUIX_CHECK_GUILE_SSH
 dnl
 dnl Check whether a recent-enough Guile-SSH is available.
 AC_DEFUN([GUIX_CHECK_GUILE_SSH], [
-  dnl Check whether 'channel-send-eof' (introduced in 0.10.2) is present.
+  dnl Check whether 'userauth-gssapi!' (introduced in 0.12.0) is present.
   AC_CACHE_CHECK([whether Guile-SSH is available and recent enough],
     [guix_cv_have_recent_guile_ssh],
     [GUILE_CHECK([retval],
       [(and (@ (ssh channel) channel-send-eof)
             (@ (ssh popen) open-remote-pipe)
-	    (@ (ssh dist node) node-eval))])
+            (@ (ssh dist node) node-eval)
+            (@ (ssh auth) userauth-gssapi!))])
      if test "$retval" = 0; then
        guix_cv_have_recent_guile_ssh="yes"
      else