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) (disable for the moment)
2 jobs for master in 15 minutes and 19 seconds (queued for 60 minutes and 19 seconds)
Status Job ID Name Coverage
  Build
passed #82697
build_clazy_clang_tidy

00:09:38

passed #82696
linux kf5-qt5 qt5.15

00:05:40

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