[CMake] Don't download external projects again after calling "make clean"

[Cedric Doucet]

Hello! 

I use the ExternalProject_Add function to download third-party libraries of a code. 

Once a library has been downloaded, I can call "make" as many times as I want without downloading this library again. 
It seems that CMake detects that the library has already been downloaded. 

However, calling "make clean" seems to destroy this feature. 
Even if my library is not uninstalled during cleaning, calling "make" after "make clean" will lead CMake to try download the library again. 

How could I tell CMake not to download the library again? 

Thank you very much for your help! 

Cédric 
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