configure: disable libidn by default

For security reasons, until there is a fix.

Bug: http://curl.haxx.se/mail/lib-2015-06/0143.html
Reported-by: Gustavo Grieco, Feist Josselin

configure.ac

@@ -2659,9 +2659,9 @@ case "$OPT_IDN" in
     ;;
   default)
     dnl configure option not specified
-    want_idn="yes"
+    want_idn="no"
     want_idn_path="default"
-    AC_MSG_RESULT([(assumed) yes])
+    AC_MSG_RESULT([(assumed) no])
     ;;
   yes)
     dnl --with-libidn option used without path