Log Message:
net/net6: Fix regression due to upgrade to gnutils 3.0

GnuTLS deprecated gnutils_transport_set_lowat function in version 2.12.0
and finally removed it with version 3.0, breaking any packages that
still reference it.

The lowat feature is now disabled permanently I think.
The patch uses the GNUTLS_VERSION_NUMBER macro to appropriately conceal
the function reference.  The same patch is widely seen on the 'net with
other packages that use GnuTLS.