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 29 minutes and 44 seconds (queued for 29 minutes and 55 seconds)
Status Job ID Name Coverage
  Build
failed #82693
build_clazy_clang_tidy

00:16:35

passed #82692
linux kf5-qt5 qt5.15

00:13:09

36.0%
 
Name Stage Failure
failed
build_clazy_clang_tidy Build
make[2]: *** [resources/imap/autotests/CMakeFiles/testaddcollectiontask.dir/build.make:165: bin/testaddcollectiontask] Error 1
make[1]: *** [CMakeFiles/Makefile2:3495: resources/imap/autotests/CMakeFiles/testaddcollectiontask.dir/all] Error 2
make: *** [Makefile:160: all] Error 2
Uploading artifacts for failed job
Uploading artifacts...
WARNING: JUnitTestResults.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1