With add_custom_target, users can execute custom commands within targets. This is subtly different from the add_custom_command recipe we have discussed previously. The target added by add_custom_target has no output and is thus always executed. It is thus possible to introduce a custom target in subdirectories, and still be able to refer to it in the top-level CMakeLists.txt.
In this example, we have extracted an archive of source files using a combination of add_custom_target and add_custom_command. These source files were later used to compile a library that we managed to link against in a different (parent) directory scope. In the construction of the CMakeLists.txt files, we briefly commented that the tarball is extracted ...
