Standalone Build Fix, master branch (2019.05.16.)

Made the LibLZMA dependency in GaudiKernel optional. To allow the usage of hand-built ROOT versions, ones that were not built against LibLZMA.

At the same time I just now noticed that we can no longer use GCC 7 for the build. I guess this was already known, but I just wanted to make sure that everybody's aware. I can use GCC 8 on Ubuntu 18.04 (where I was doing this test) relatively conveniently. But I was still surprised that the system-default compiler no longer works with Gaudi...