[Perl/perl5] f9624f: Add ability to easily add info to DEBUG output

[Karl Williamson via perl5-changes]

  Branch: refs/heads/khw/pr3
  Home:   https://github.com/Perl/perl5
  Commit: f9624f0ba84a37d08a717932165977fdeef4fe22
      
https://github.com/Perl/perl5/commit/f9624f0ba84a37d08a717932165977fdeef4fe22
  Author: Karl Williamson <k...@cpan.org>
  Date:   2021-02-24 (Wed, 24 Feb 2021)

  Changed paths:
    M perl.h

  Log Message:
  -----------
  Add ability to easily add info to DEBUG output

This commit adds two macros that a user can define and recompile Perl to
get every active DEBUG statement to do  something beyond what it would
normally do.

This allows someone to recompile Perl when they need to delve deeper
into fixing a bug without increasing memory use or slowing execution
otherwise.