Fixed compilation in systems without EPOLL
Kill compilation warnings

io_wait.c: In function ‘check_poll_method’:
io_wait.c:368:15: warning: variable ‘os_ver’ set but not used [-Wunused-but-set-variable]
  unsigned int os_ver;
               ^
io_wait.c: In function ‘choose_poll_method’:
io_wait.c:448:15: warning: variable ‘os_ver’ set but not used [-Wunused-but-set-variable]
  unsigned int os_ver;
               ^