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