pack "D" for ldouble not available for all Perl

[zmughal]

$ use PDL;
$ ldouble(0)
Runtime error: Invalid type 'D' in pack at /perls/perl-5.26.1/lib/site_perl/5.26.1/x86_64-linux/PDL/Core.pm line 1211.

But it will be available on more Perls with Perl/perl5#18517.

Perhaps a similar fix to

sub PDL::new {
# ...
      } elsif ($Config{ivsize} < 8 && $pack[$new->get_datatype] eq 'q*') {
         # special case when running on a perl without 64bit int support
         # we have to avoid pack("q", ...) in this case
         # because it dies with error: "Invalid type 'q' in pack"
         $new->setdims([]);
         set_c($new, [0], $value);
# ...

is needed.

[mohawk2]

Great work @zmughal! We should probably document this limitation, and PDL::new looks like it might benefit from a proper review.