Check if libcurl was built with async DNS support

info.c

#include <stdio.h>
#include <curl/curl.h>

int
main(void)
{
  curl_version_info_data *info = curl_version_info(CURLVERSION_NOW);
  printf("async DNS: %d\n", !!(info->features & CURL_VERSION_ASYNCHDNS));
  return 0;
}