configure.ac: Link test against pthread_atfork() instead of pthread_create().
configure.ac: Link test against pthread_atfork() instead of pthread_create().

On some platforms (e.g. CentOS 5), linking against -lrt makes
available for linking some pthreads symbols (e.g. pthread_create)
but not others (e.g. pthread_atfork).  Do the link test against
pthread_atfork() instead of pthread_create() for that reason.

Signed-off-by: Lennert Buytenhek <8858498612e3a522354216649776e3032dcb7e5b@wantstofly.org>

buytenh/ivykis,buytenh/ivykis