The detection for Ubuntu 16.04 was broken. By now /etc/os-release contains VERSION="16.04.7 LTS (Xenial Xerus)"
On Ubuntu 16.04 (trusty) valgrind fails due to rdrand being advertised but not implemented. Work around that by installing valgrind from Ubuntu 18.04 (bionic) via the "contrib/scripts/nm-ci-install-valgrind-in-ubuntu1604.sh" script.