[CMake] cmake - preventing `make clean` from cleaning ExternalProject.

[Rolf Eike Beer]

> I was wondering if there's some way to prevent make clean in cmake from
> re-building external dependencies. I'm using ExternalProject to build
> third
> party c++ libraries, and they do not have to be rebuilt even if I do make
> clean.
>
> On the other hand, I might want to create a new rule, say, make
> really-clean,
> which even clears the dependencies. is there a good way to do this?

The natural choices for the name of this target would be "distclean" and
"mrproper".

Eike