explicitly chose generational GC

Parrot folks say that it will probably not be the default for the next
release, so there is no harm in selecting it explicitly.
tadzik++ for the idea

Configure.pl

@@ -26,7 +26,8 @@
         my $prefix  = $options{'gen-parrot-prefix'} || cwd()."/parrot_install";
         # parrot's Configure.pl mishandles win32 backslashes in --prefix
         $prefix =~ s{\\}{/}g;
-        my @command = ($^X, "build/gen_parrot.pl", "--prefix=$prefix", ($^O !~ /win32/i ? "--optimize" : ()), @opts);
+        my @command = ($^X, "build/gen_parrot.pl", "--prefix=$prefix",
+                '--gc=gms', ($^O !~ /win32/i ? "--optimize" : ()), @opts);
 
         print "Generating Parrot ...\n";
         print "@command\n\n";