RT #132834: prevent duplicate Config.pm vars

Commit 6c2ae64 introduced variables whose
need was forced by configpm (for compatibility with older software), but
incorrectly added those variables to the generated Config_heavy.pl even when
they were already in use.

The resulting duplicate variables are clearly wrong, and in addition they
broke ExtUtils::InferConfig.

(cherry picked from commit 8e4ae0f)

configpm

@@ -595,9 +595,11 @@ local *_ = \my $a;
 $_ = <<'!END!';
 EOT
 #proper lexicographical order of the keys
+my %seen_var;
 $heavy_txt .= join('',
     map { $_->[-1] }
     sort {$a->[0] cmp $b->[0] }
+    grep { !$seen_var{ $_->[0] }++ }
     map {
         /^([^=]+)/ ? [ $1, $_ ]
                    : [ $_, $_ ] # shouldnt happen