ParseXS: 5.8.9 backcompat fixes

A couple of fixes to make it build+test under 5.8.9.

The grep expression

    exists $_->{in_out} && $_->{in_out} =~ /OUT$/

was wrong - it should have been defined rather than exists, as is done
elsewhere. I'm not sure why an 'uninit var' warning only appeared on 5.8.9
but not blead - perhaps some minor autovivification difference?

It was also warning

    $ExtUtils::ParseXS::DIE_ON_ERROR only used once
    $ExtUtils::ParseXS::AUTHOR_WARNINGS only used once

in t/001-basic.t because that test file only loads ExtUtils::ParseXS at
runtime. Again, I'm not sure why it didn't warn on blead too. But I made
the var initialisations more robust against 'once' warnings anyway by
using 'our'.

Author: iabyn

dist/ExtUtils-ParseXS/lib/ExtUtils/ParseXS.pm

@@ -1250,7 +1250,7 @@ EOF
 
       for my $param (
               grep {
-                     exists $_->{in_out}
+                     defined $_->{in_out}
                   && $_->{in_out} =~ /OUT$/
                   && !$self->{xsub_map_varname_to_seen_in_OUTPUT}{$_->{var}}
               }

dist/ExtUtils-ParseXS/t/001-basic.t

@@ -29,8 +29,10 @@ require_ok( 'ExtUtils::ParseXS' );
 chdir('t') if -d 't';
 push @INC, '.';
 
-$ExtUtils::ParseXS::DIE_ON_ERROR = 1;
-$ExtUtils::ParseXS::AUTHOR_WARNINGS = 1;
+package ExtUtils::ParseXS;
+our $DIE_ON_ERROR = 1;
+our $AUTHOR_WARNINGS = 1;
+package main;
 
 use Carp; #$SIG{__WARN__} = \&Carp::cluck;