Idea is to wait for a falling edge on the signal at pin2, the ISR sets syncFlag = true; and the while loop falls through.
This works perfectly using IDE 1.6.5. When I updated to IDE 1.8.1 it no longer works. The program appears to never fall through the while loop. If I comment out the while statement the program runs as expected, but of course is no longer synchronized to the signal on pin 2.
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You must declare any variable that can be changed in an ISR as volatile. Otherwise, the compiler doesn't know that it has to check the value of that variable each time you use it, rather than fetching it once and reusing the value (which is typically faster and smaller, and hence an appealing target for optimization).
The optimization in recent IDE versions is a little better (actually, it's a LOT better - take a look at the size of a sketch under 1.6.5 vs 1.8.1), and it's not missing the chance to optimize that while loop anymore.