Major hostip.c cleanup and split into multiple files and easier #ifdef

usage.

lib/setup.h

@@ -261,9 +261,13 @@ typedef int curl_socket_t;
 #error "ares does not yet support IPv6. Disable IPv6 or ares and rebuild"
 #endif
 
-#if defined(WIN32) && !defined(__CYGWIN32__) && !defined(USE_ARES) && !defined(ENABLE_IPV6)
+#if defined(WIN32) && !defined(__CYGWIN__) && !defined(USE_ARES)
+#ifdef ENABLE_IPV6
+#define USE_THREADING_GETADDRINFO
+#else
 #define USE_THREADING_GETHOSTBYNAME  /* Cygwin uses alarm() function */
 #endif
+#endif
 
 /*
  * Curl_addrinfo MUST be used for name resolving information.
@@ -296,4 +300,10 @@ typedef struct in_addr Curl_ipconnect;
 #undef HAVE_ALARM
 #endif
 
+#ifdef HAVE_LIBIDN
+/* 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
+#endif
+
 #endif /* __CONFIG_H */