Merge MinGW surrogate build fix from pre-0.64.

putty.h

@@ -1442,12 +1442,14 @@ void request_callback_notifications(toplevel_callback_notify_fn_t notify,
 #endif
 
 /* SURROGATE PAIR */
-#ifndef IS_HIGH_SURROGATE
 #define HIGH_SURROGATE_START 0xd800
 #define HIGH_SURROGATE_END 0xdbff
 #define LOW_SURROGATE_START 0xdc00
 #define LOW_SURROGATE_END 0xdfff
 
+/* These macros exist in the Windows API, so the environment may
+ * provide them. If not, define them in terms of the above. */
+#ifndef IS_HIGH_SURROGATE
 #define IS_HIGH_SURROGATE(wch) (((wch) >= HIGH_SURROGATE_START) && \
                                 ((wch) <= HIGH_SURROGATE_END))
 #define IS_LOW_SURROGATE(wch) (((wch) >= LOW_SURROGATE_START) && \