Migrate off of posix_time and most uses of C time_t.

src/ripple/server/impl/JSONRPCUtil.cpp

@@ -36,10 +36,15 @@ std::string getHTTPHeaderTimestamp ()
     char buffer[96];
     time_t now;
     time (&now);
-    struct tm* now_gmt = gmtime (&now);
+    struct tm now_gmt{};
+#ifndef _MSC_VER
+    gmtime_r(&now, &now_gmt);
+#else
+    gmtime_s(&now_gmt, &now);
+#endif
     strftime (buffer, sizeof (buffer),
         "Date: %a, %d %b %Y %H:%M:%S +0000\r\n",
-        now_gmt);
+        &now_gmt);
     return std::string (buffer);
 }