Fix cert expiry warning for Windows, which lacks gmtime_r()

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

gnutls.c

@@ -210,12 +210,27 @@ static int check_certificate_expiry(struct openconnect_info *vpninfo, gnutls_x50
 		reason = _("Client certificate expires soon at");
 
 	if (reason) {
-		struct tm tm;
 		char buf[80];
+#ifdef _WIN32
+		/*
+		 * Windows doesn't have gmtime_r but apparently its gmtime()
+		 * *is* thread-safe because it uses a per-thread static buffer.
+		 * cf. http://sourceforge.net/p/mingw/bugs/1625/
+		 *
+		 * We also explicitly say 'GMT' because %Z would give us the
+		 * Microsoft stupidity "GMT Standard Time". Which is not only
+		 * silly, but also ambiguous because Windows actually says that
+		 * even when it means British Summer Time (GMT+1). And having
+		 * used gmtime() we really *are* giving the time in GMT.
+		 */
+		struct tm *tm = gmtime(&expires);
+		strftime(buf, 80, "%a, %d %b %Y %H:%M:%S GMT", tm);
+#else
+		struct tm tm;
 
 		gmtime_r(&expires, &tm);
 		strftime(buf, 80, "%a, %d %b %Y %T %Z", &tm);
-
+#endif
 		vpn_progress(vpninfo, PRG_ERR, "%s: %s\n", reason, buf);
 	}
 	return 0;