Change 30461 was wrong. As ext/re (re)builds the regexp engine with

Change 30461 was wrong. As ext/re (re)builds the regexp engine with
-DDEBUGGING, it's going to need PL_reg_name even if core perl doesn't.
So something is always going to use it, so always define it, and always
export it. (But only define it once, so that static builds work.)

p4raw-id: //depot/perl@30464