[CMake] rebuild_cache does not fully rebuild the cache information

[Nils Rathmann]

Hi,
when I change the installation of external dependencies, which are 
located by find_packages, it seems, that it is not enough to run 
rebuild_cache. The information about external software is not thrown 
away and rebuild. I have to delete the CMakeCache.txt and rebuild again.
It this intended? And why is it intended?

I work with cmake 3.9.1.

Thank you,
Nils