To be accurate, we run the standalone-qt project which uses the same superbuild code to build and install qt5 on the machine and then we build against that version of Qt to save time. (Not rebuilding Qt cuts build time roughly in half). Also our buildbot is CentOS 6 with newer compilers, so it may be an Ubuntu problem.
Have you tried changing the qt5_SOURCE_SELECTION cmake variable? Unless you need Qt 5.8 for some reason, you could try building with 5.9 or 5.10. Ubuntu 18.04 is very new and you may have better luck with a newer Qt version.