Allow to use UNITY build see...

Allow to use UNITY build see https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html (Reduce compile time. Need cmake 3.16.0)
2 jobs for master in 16 minutes and 4 seconds (queued for 71 minutes and 18 seconds)
Status Job ID Name Coverage
  Build
passed #82422
build_clazy_clang_tidy

00:09:55

passed #82421
linux kf5-qt5 qt5.15

00:06:08

45.0%