CURL_EXTERN is already in <curl/curl.h>.

ares/ares_private.h

@@ -164,9 +164,5 @@ int ares__read_line(FILE *fp, char **buf, int *bufsize);
    libcurl lowlevel code from within library is ugly and only works when
    c-ares is built and linked with a similarly debug-build libcurl, but we do
    this anyway for convenience. */
-#ifndef CURL_EXTERN
-/* ugly hack to make this compile */
-#define CURL_EXTERN
-#endif
 #include "../lib/memdebug.h"
 #endif