Support LDAP GSSAPI authentication (#5703)

Requires https://git.kolab.org/rPNL1d87f98d300645e15b7ad2819fcffb19010a060f

CHANGELOG

@@ -1,6 +1,7 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Support LDAP GSSAPI authentication (#5703)
 - Allow contacts without an email address (#5079)
 - Localized timezone selector (#4983)
 - Update to TinyMCE 4.5.7

program/lib/Roundcube/rcube_ldap.php

@@ -285,6 +285,7 @@ class rcube_ldap extends rcube_addressbook
             $bind_pass   = $this->prop['bind_pass'];
             $bind_user   = $this->prop['bind_user'];
             $bind_dn     = $this->prop['bind_dn'];
+            $auth_method = $this->prop['auth_method'];
 
             $this->base_dn        = $this->prop['base_dn'];
             $this->groups_base_dn = $this->prop['groups']['base_dn'] ?: $this->base_dn;
@@ -420,7 +421,7 @@ class rcube_ldap extends rcube_addressbook
                 }
             }
 
-            if (empty($bind_pass)) {
+            if (empty($bind_pass) && strncasecmp($method, 'GSSAPI', 6) != 0) {
                 $this->ready = true;
             }
             else {