How is JUCE_IS_REPAINT_DEBUGGING_ACTIVE supposed to work?

I assume I do something like:

#define JUCE_IS_REPAINT_DEBUGGING_ACTIVE g_paintDebugging

but then where do I define g_paintDebugging so that I can access it from my code and the component peer implementation can see it?

I use it in Projucer - in the juce_gui_basics module there is an option:- JUCE_ENABLE_REPAINT_DEBUGGING.
It draws a coloured box over whatever is being drawn. I’ve found it useful in optimising draws at times.

I’m talking about this commit here:

It’s to enable dynamic debug repainting. But no explanation on how it’s supposed to work.

Bump. Can anybody from the JUCE team explain how this is supposed to work?