Support build with older libssh.

The function ssh_session_is_known_server() was introduced later,
fallback to older version if libssh is available.

configure.ac

@@ -402,6 +402,7 @@ AC_CHECK_DECLS([json_object_deep_copy], [], [], [#include <json-c/json.h>])
 dnl Check for libssh for SSH plugin
 if test "x$enable_ssh_token" = "xyes"; then
 	PKG_CHECK_MODULES([LIBSSH], [libssh])
+	AC_CHECK_DECLS([ssh_session_is_known_server], [], [], [#include <libssh/libssh.h>])
 fi
 
 dnl Crypto backend configuration.

tokens/ssh/ssh-utils.c

@@ -123,7 +123,11 @@ ssh_session sshplugin_session_init(struct crypt_device *cd, const char *host, co
 		goto out;
 	}
 
+#if HAVE_DECL_SSH_SESSION_IS_KNOWN_SERVER
 	r = ssh_session_is_known_server(ssh);
+#else
+	r = ssh_is_server_known(ssh);
+#endif
 	if (r != SSH_SERVER_KNOWN_OK) {
 		crypt_log(cd, CRYPT_LOG_ERROR, _("Server not known: "));
 		r = SSH_AUTH_ERROR;