build: compile websocketpp in C++11

Change-Id: Iad6a34f2b081030c4806ecc2b8f732206bd80d7a
Refs: #1930, #2082
diff --git a/.waf-tools/websocket.py b/.waf-tools/websocket.py
index c5b3384..f8d2e33 100644
--- a/.waf-tools/websocket.py
+++ b/.waf-tools/websocket.py
@@ -59,10 +59,10 @@
 
         self.end_msg('.'.join(self.env['WEBSOCKET_VERSION']))
 
-
         self.env['INCLUDES_WEBSOCKET'] = websocketDir.parent.abspath()
         self.env['HAVE_WEBSOCKET'] = True
         self.define('HAVE_WEBSOCKET', 1)
+        self.define('_WEBSOCKETPP_CPP11_STL_', 1)
 
     except Errors.WafError as error:
         if isMandatory: