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 8 minutes and 39 seconds (queued for 128 minutes and 36 seconds)
Status Job ID Name Coverage
  Build
passed #82428
build_clazy_clang_tidy

00:04:55

passed #82427
linux kf5-qt5 qt5.15

00:03:44

16.0%