what does add_dependence() do?

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According to add_dependencies's documentation:

Add a dependency between top-level targets.

add_dependencies(<target> [<target-dependency>]...)

Make a top-level <target> depend on other top-level targets to ensure that they build before <target> does. A top-level target is one created by ADD_EXECUTABLE, ADD_LIBRARY, or ADD_CUSTOM_TARGET. Dependencies added to an IMPORTED target are followed transitively in its place since the target itself does not build.

See the DEPENDS option of ADD_CUSTOM_TARGET and ADD_CUSTOM_COMMAND for adding file-level dependencies in custom rules. See the OBJECT_DEPENDS option in SET_SOURCE_FILES_PROPERTIES to add file-level dependencies to object files.