Simon Josefsson added a idn_free() function in libidn 0.4.5 as a reaction to

Gisle's previous mail. We now use this function, and thus we require libidn
 0.4.5 or later. No earler version will do.

lib/setup.h

@@ -304,6 +304,7 @@ typedef struct in_addr Curl_ipconnect;
 /* This could benefit from additional checks that some of the used/important
    header files are present as well before we define the USE_* define. */
 #define USE_LIBIDN
+#define LIBIDN_REQUIRED_VERSION "0.4.5"
 #endif
 
 #endif /* __CONFIG_H */