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 7 minutes and 51 seconds (queued for 167 minutes and 37 seconds)
Status Job ID Name Coverage
  Build
passed #82502
build_clazy_clang_tidy

00:04:13

passed #82501
linux kf5-qt5 qt5.15

00:03:37

69.0%