Harmonised CMakeLists.txt

I don't know if this is important or just some cosmetics, but the current master CMakeLists.txt contains farraginous indentation, stray trailing whitespaces, irregular structures and so on.

This patch does not change anything of the content, it just unifies the indentation (always four spaces, as requested for the KPA code), unifies e. g. the way source files are added, removes trailing whitespace and so on.

I think this rework would not be bad to be added to master, as simply adding a line to CMakeLists.txt and saving it with an editor discarding trailing whitespace changed the file all over.