gtls: only call gnutls_transport_set_lowat with <gnutls-2.12.0

The default lowat level for gnutls-2.12* is set to zero to avoid
unnecessary system calls and the gnutls_transport_set_lowat function has
been totally removed in >=gnutls-3 which causes build failures.

Therefore, the function shouldn't be used except for versions that
require it, <gnutls-2.12.0.