Log Message:
libgtop: fix build with Perl >= 5.30

As of Perl 5.30, it's no longer permitted to set $[ to a non-zero
value, which was causing the lib.pl script to fail. Adjust the script
to use zero-based indexing. (Upstream has also changed this script
accordingly in subsequent releases, but this isn't a straight lift from
there, as the ingoing features.def input file format has also changed
in intervening releases, and there were other, unrelated changes
applied too.) This is a workaround until this package is updated to a
newer release (that is non-trivial because of the number of local
patches, etc.). Addresses PR pkg/54475.