[CMake] External projects and make clean

[Oliver Boesche]

Hi,

I use external projects e.g. to build BOOST for my own project. In that 
case I want to prevent the project to be system or version dependent.

So I build my dependencies successful with external project (and its 
great), but if use make clean it will clean all stuff and I have to 
rebuild the external projects again (take some time with BOOST).

Is there a solution to prevent an external project from cleaning when I 
call 'make clean'?

Kind regards

Oliver Boesche