extconf.rb

require 'mkmf'
$LIBS << " -pthread"
create_makefile('myext')

nyext.c

#include <pthread.h>
#include <stdlib.h>

static void *dummy(void *arg) {
        reurn NULL;
}

void Init_myext() {
        pthread_t thr;
        pthread_create(&thr, NULL, dummy, NULL);
}