CI: build with CGO_ENABLED=0 on ubuntu1604
CI: build with CGO_ENABLED=0 on ubuntu1604

libc on Ubuntu 16.04 is too old for golang 1.12 ->
build gateway without linking to libc

DrDaveD/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs