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 2 minutes and 34 seconds (queued for 76 minutes and 47 seconds)
Status Job ID Name Coverage
  Build
passed build_clazy_clang_tidy #82561

00:01:33

passed linux kf5-qt5 qt5.15 #82560

00:01:00

69.0%