Work around lack of SIGCHLD on Windows

This only enables the SIGCHLD handler if SIGCHLD
exists, such that platforms without SIGCHLD (such
as windows) can still run websockify natively.

See #108

websockify/websocket.py

@@ -913,10 +913,14 @@ class WebSocketServer(object):
         original_signals = {
             signal.SIGINT: signal.getsignal(signal.SIGINT),
             signal.SIGTERM: signal.getsignal(signal.SIGTERM),
-            signal.SIGCHLD: signal.getsignal(signal.SIGCHLD),
         }
+        if getattr(signal, 'SIGCHLD', None) is not None:
+            original_signals[signal.SIGCHLD] = signal.getsignal(signal.SIGCHLD),
+
         signal.signal(signal.SIGINT, self.do_SIGINT)
         signal.signal(signal.SIGTERM, self.do_SIGTERM)
+
+        if getattr(signal, 'SIGCHLD', None) is not None:
             if not multiprocessing:
                 # os.fork() (python 2.4) child reaper
                 signal.signal(signal.SIGCHLD, self.fallback_SIGCHLD)