start building thread support for lang/perl5.12 by default

[Emanuel Haupt]

May I suggest to start enabling thread support for lang/perl5.12 by
default. While thread support was considered unstable for a long time
it has become more stable with perl 5.12. While some of us might
consider perl threads still unstable it shouldn't cause any harm if
threads arent imported.

Finally, many Linux distributions have started to ship perl with thread
support by default (eg. RHEL5).

Emanuel