tests: virnettlscontexttest needs gnutls-2.6.0

virnettlscontexttest uses gnutls_x509_crt_set_subject_alt_name() and
GNUTLS_FSAN_APPEND, which - according to
<http://www.gnu.org/software/gnutls/manual/gnutls.html> - are only
available since 2.6.0.

Since libvirt still works fine with gnutls-1.0.25 from RHEL5, only
enable the test when the version of GNUTLS is at least 2.6.0.
Signed-off-by: NPhilipp Hahn <hahn@univention.de>
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/virnettlscontexttest.c

 /*
- * Copyright (C) 2011 Red Hat, Inc.
+ * Copyright (C) 2011-2012 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -36,7 +36,7 @@
 #include "virsocketaddr.h"
 #include "gnutls_1_0_compat.h"
 
-#if !defined WIN32 && HAVE_LIBTASN1_H && !defined GNUTLS_1_0_COMPAT
+#if !defined WIN32 && HAVE_LIBTASN1_H && LIBGNUTLS_VERSION_NUMBER >= 0x020600
 # include <libtasn1.h>
 
 # include "rpc/virnettlscontext.h"