Unlike GNU make, ninja (https://ninja-build.org/) needs to know which
target produces the named output. Without marking the output libraries
as byproducts ninja does not know that it is a file that will be
created later on by one of the targets and assumes it missing.

Without the dependency, CMake runs this command at every build which
leads to unnecessary recompilation of dummy.cpp.

Now that the call is handled in CMake, this step is not required
anymore.