s4:lib/tls - call "gnutls_transport_set_lowat" only on GNUTLS < 3.0

s4:lib/tls - call "gnutls_transport_set_lowat" only on GNUTLS < 3.0

This function call together with the lowat feature has been removed in release
3.0 as described in this mailing list post:
http://old.nabble.com/gnutls_transport_set_lowat-deprecated-td32554230.html.

Since we do not make any use of lowat (esprimed by each function call)
we are free to simply omit it on v3.0 and later.

This addresses bug #8537.

Reviewed by: abartlet + metze

Autobuild-User: Matthias Dieter Wallnöfer <mdw@samba.org>
Autobuild-Date: Wed Nov 30 20:11:14 CET 2011 on sn-devel-104