gai_strerror: provide private implementation

There are systems (like NetWare) without its own gai_strerror()
function.

lib/asyn-thread.c

@@ -415,6 +415,44 @@ static bool init_resolve_thread (struct connectdata *conn,
   return FALSE;
 }
 
+#if defined(HAVE_GETADDRINFO) && !defined(HAVE_GAI_STRERROR) && !defined(WIN32)
+/* NetWare has getaddrinfo but lacks gai_strerror.
+   Windows has a gai_strerror but it is bad (not thread-safe) and the generic
+   socket error string function can be used for this pupose. */
+static const char *gai_strerror(int ecode)
+{
+  switch (ecode){
+  case EAI_AGAIN:
+    return "The name could not be resolved at this time";
+  case EAI_BADFLAGS:
+    return "The flags parameter had an invalid value";
+  case EAI_FAIL:
+    return "A non-recoverable error occurred when attempting to "
+      "resolve the name";
+  case EAI_FAMILY:
+    return "The address family was not recognized";
+  case EAI_MEMORY:
+    return "Out of memory";
+  case EAI_NONAME:
+    return "The name does not resolve for the supplied parameters";
+  case EAI_SERVICE:
+    return "The service passed was not recognized for the "
+      "specified socket type"
+  case EAI_SOCKTYPE:
+    return "The intended socket type was not recognized"
+  case EAI_SYSTEM:
+    return "A system error occurred";
+  case EAI_OVERFLOW:
+    return "An argument buffer overflowed";
+  default:
+    return "Unknown error";
+
+/* define this now as this is a private implementation of said function */
+#define HAVE_GAI_STRERROR
+}
+#endif
+
+
 /*
  * resolver_error() calls failf() with the appropriate message after a resolve
  * error