*) All extern functions which an implementation must or may want to (for
some funcitons) override are now declared in
lib/clock_extern_functions.h and stubbed to NULL by default

+) lib/clock_extern_functions.c::clock_initDateTime(DateTime *) an
implementation may define this function. If it is not NULL,
lib/clock_main.c::clock_init(ClockState *) will call it to initialize
ClockState->DateTime. I.e. an implementation may define the function to
set the date and time to current date and time