#include <features.h>
#if defined(__GLIBC__) && (__GLIBC__ >= 2) && (__GLIBC__ + __GLIBC_MINOR__ >= 3)
  /* For IPv6 support, glibc2.1 or newer is required */
  echo 1
#else
  /* Sorry, no IPv6 support */
  echo 0
#endif