The file apr_network_io.h defines a MACRO named APR_INET6 in case feature APR_HAVE_IPV6 is enabled. This MACRO is used in file ab.c, but there is no #ifdef APR_HAVE_IPV6 encompassing such a use.

Bug fixed by commit 581635e28f1

#if APR_HAVE_IPV6
#define APR_INET6 6
#endif

int parse_url()
{
  int family = APR_INET6;
}

int main(void)
{
  parse_url();
  return 0;
}

. // if APR_HAVE_IPV6 is disabled
. ERROR support/ab.c:972:if (family == APR_INET6)