[CMake] cmake install behaviour with git

[Jörg Kreuzberger]

> What was unclear to me is that it now sounds like you are installing two 
> distinct files from your repository to the same destination file rather 
> than re-installing a modified version of the same file.
Yes, your are right i am doing exactly this

> I think that is conceptually an error prone thing to do irregardless of 
> the SCM in use.
yes you are right, but it worked until the git change :-)
i will fix it in your CMakeLists.txt