[CMake] Alias of imported target

[Stephen Kelly]

Ruslan Baratov via CMake wrote:

> It may be helpful in situations when you build package with examples. In
> case when you want examples to work both as a part of a project
> (add_subdirectory) or stand-alone (find_package).

I expect external_project may lead to more readability for this case, though 
the case you described also makes sense.

Thanks,

Steve.