Mention OPTIMIZE=0 more prominently in the hacking guide

[thufschmitt]

This is a game-changer when developing, it shouldn't just be hidden amongst a list of more advanced variables.

Note that it's currently broken (#9965), so keeping it as a draft for now.

Depends on #9965

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[tomberek]

This sometimes has no effect as it is easy to end up with a call to g++ ... -O2 .... -O0 .... -O2 with opt flags overriding each other. I didn't trace this issue to a cause, but noticed that OPTIMIZE=0 sometimes has no effect.

[thufschmitt]

This sometimes has no effect as it is easy to end up with a call to g++ ... -O2 .... -O0 .... -O2 with opt flags overriding each other. I didn't trace this issue to a cause, but noticed that OPTIMIZE=0 sometimes has no effect.

Yes, that's what #9965 mentions. I couldn't find how to change that (seems to be some builtin autoconf behaviour), hoping for someone more knowledgeable to chime in