Delays (Sleep, sleep, usleep) definitions

It raises an error saying that there are no reference for:
sleep(), usleep()... etc 

This should be resolved by including <unistd.h> in the right header files (for *nix) or <windows.h>, and check for the current OS to specify when to use <windows.h> Sleep() or <unistd.h> sleep() (or even writing a macro for that).
