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 like OpenVAS.
