Add wxUSE_WINSOCK2 wxMSW option to include winsock2.h

Including <winsock.h> and <winsock2.h> is incompatible and if the
application wants to use the latter, it may be convenient to define
wxUSE_WINSOCK2 when building wxWidgets instead of having to work around
winsock.h implicit inclusion from include/wx/msw/wrapwin.h.

Closes https://github.com/wxWidgets/wxWidgets/pull/1122

include/wx/msw/chkconf.h

@@ -118,6 +118,14 @@
 #    endif
 #endif  /* wxUSE_UXTHEME */
 
+#ifndef wxUSE_WINSOCK2
+#    ifdef wxABORT_ON_CONFIG_ERROR
+#        error "wxUSE_WINSOCK2 must be defined."
+#    else
+#        define wxUSE_WINSOCK2 0
+#    endif
+#endif  /* wxUSE_WINSOCK2 */
+
 /*
  * Unfortunately we can't use compiler TLS support if the library can be used
  * inside a dynamically loaded DLL under Windows XP, as this can result in hard
@@ -455,4 +463,14 @@
 #   define wxUSE_POSTSCRIPT 1
 #endif
 
+/*
+    IPv6 support requires winsock2.h, but the default of wxUSE_WINSOCK2 is 0.
+    Don't require changing it explicitly and just turn it on automatically if
+    wxUSE_IPV6 is on.
+ */
+#if wxUSE_IPV6 && !wxUSE_WINSOCK2
+    #undef wxUSE_WINSOCK2
+    #define wxUSE_WINSOCK2 1
+#endif
+
 #endif /* _WX_MSW_CHKCONF_H_ */