Rules for inlude files:
- include files from the current subdir via #include "abc.h"
- include header from other subdirs vai #include <def.h>
  ( do NOT use a specific path - this may change!)
- include privtae header files via path: #include "../other_dir/ppp.h"
  - should be avoided!

Classes of files
- public header:
  - should be installed at a central place ($includedir)
  - should itself only include other public headers
- private header: 
  - will not be installed at other places
  - will (usually) only be included from that subdir