[time-nuts] verifying synchronization with PPS

[folkert]

> > Folkert van Heusden has a driver for NTP which includes PPS output:
> >  https://vanheusden.com/time/rpi_gpio_ntp/
> > Perhaps this might help?
> 
> Indeed I did! :-)
> 
> But please note that the jitter is high, iirc around 18ms.
> Personally I would use https://github.com/mlichvar/pps-gpio-poll.git and
> then patch it toggle an other gpio-pin.

It has this functionality, thus no real patching required.
You only need to remove the /* around #define GPIO_ECHO in
pps-gpio-poll.c and then make/install/reboot (well not literally reboot
of course).