Added CURL_CHECK_NI_WITHSCOPEID that checks if NI_WITHSCOPEID exists and

works. No code actually uses the HAVE_NI_WITHSCOPEID (that a positive test
results in), but this is still only for testing purposes.

configure.ac

@@ -384,6 +384,8 @@ main()
 
 if test "$ipv6" = "yes"; then
   CURL_CHECK_WORKING_GETADDRINFO
+
+  CURL_CHECK_NI_WITHSCOPEID
 fi
 
 dnl **********************************************************************