windows: enable UnixSockets with all build toolchains

Extend existing unix socket support in Windows builds to be
enabled for all toolchain vendors or versions. (Previously
it was only supported with certain MSVC versions + more recent
Windows 10 SDKs)

Ref: https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/
Ref: #5162
Closes: #5170

lib/config-win32.h

@@ -719,12 +719,15 @@ Vista
 #endif
 
 /* Define to use Unix sockets. */
-#if defined(_MSC_VER) && (_MSC_VER >= 1500)
-/* sdkddkver.h first shipped with Platform SDK v6.0A included with VS2008 */
-#include <sdkddkver.h>
-#if defined(NTDDI_WIN10_RS4)
 #define USE_UNIX_SOCKETS
-#endif
+#if !defined(UNIX_PATH_MAX)
+  /* Replicating logic present in afunix.h of newer Windows 10 SDK versions */
+# define UNIX_PATH_MAX 108
+# include <ws2tcpip.h>
+  typedef struct sockaddr_un {
+    ADDRESS_FAMILY sun_family;
+    char sun_path[UNIX_PATH_MAX];
+  } SOCKADDR_UN, *PSOCKADDR_UN;
 #endif
 
 /* ---------------------------------------------------------------- */

lib/curl_addrinfo.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -50,10 +50,6 @@
 #  define in_addr_t unsigned long
 #endif
 
-#if defined(WIN32) && defined(USE_UNIX_SOCKETS)
-#include <afunix.h>
-#endif
-
 #include <stddef.h>
 
 #include "curl_addrinfo.h"