Why would you use the ternary operator without assigning a value for the "true" condition (x = x ?: 1)

In the Android open-source qemu code I ran across this line of code:

machine->max_cpus = machine->max_cpus ?: 1; /* Default to UP */

Is this just a confusing way of saying:

if (machine->max_cpus) {
   ; //do nothing
} else {
 machine->max_cpus = 1;
}

If so, wouldn't it be clearer as:

if (machine->max_cpus == 0) machine->max_cpus = 1;

Interestingly, this compiles and works fine with gcc, but doesn't compile on http://www.comeaucomputing.com/tryitout/ .