ruby's fork() works good with new libpthreads

[Hleb Valoshka <375gnu@gmail.com>]

Hello!

As it was written some time ago [0] ruby has problems with fork() in
SMP environment.
But with libpthreads these problems are gone! I've made quite a lot of
tests from [0] and
there were no hangs at all. Thanks!

[0] https://lists.debian.org/debian-bsd/2012/06/msg00297.html