Use RasPi Zero to generate clock signal (for a local oscillator)

I pulled all the WSPR code from an old version of Wsprrypi to create
an application that only generates a fixed frequency with no modulation.

Why, Ed, Why?

New versions of Wsprrypi use the PWM module and that means that it 
is not available for audio out to mix with a local oscillator. 
Using this module, built-in audio, also available on the Pi Zero header,
can be mixed to provide modulated RF. Some assembly required. Combining
a PCM ADC on the receive side will implement a complete low part-count 
transceiver.

Compile by doing:
make pilo

Usage: pilo <frequency in Hz>
        e.g.: sudo ./pilo 7074000
Enter a new frequency (in Hertz) on STDIN to change the frequency.        
The clock will run until an EOF (or ^D is typed on the keyboard.)