COMP: allow Qt5 to compile on old kernels

Small compilation fix for QT5 on SuperBuild. It refers to an issue I first had on #1687 (closed). It appeared again when working on CI. The new option USE_LOW_KERNEL_VERSION can be used to disable some features that require a more recent kernel version on Linux. This will also create more portable binaries.

Typical use case: compile SuperBuild in a Ubuntu 18.04 docker running on Ubuntu 14.04 host.