Subject: | Allow building under perls with -DPERL_GLOBAL_STRUCT_PRIVATE |
cf. https://perl5.git.perl.org/perl.git/commit/8633b5394e9f1be292f2b13d5f642a205ae31509
Perl dev releases v5.29.8 and v5.29.9 currently customize vutil.c for this; it might be nice to update here instead.