configure.in: check arguments of pthread_setname_np

* configure.in (SET_THREAD_NAME): check proper arguments of
  pthread_setname_np() and fix definition on BSDs.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45202 b2dd03c8-39d4-4d8f-98ff-823fe69b080e