build: Another fix for UNIX socket detection

Change-Id: I0e9f4f191e0c3e34f1203adf7a3947923cddc94e
diff --git a/.waf-tools/unix-socket.py b/.waf-tools/unix-socket.py
index 7319816..2eea4c2 100644
--- a/.waf-tools/unix-socket.py
+++ b/.waf-tools/unix-socket.py
@@ -26,7 +26,8 @@
 
 def configure(conf):
     def boost_asio_has_local_sockets():
-        return conf.check_cxx(msg='Checking if UNIX sockets are supported',
+        return conf.check_cxx(features="cxx",
+                              msg='Checking if UNIX sockets are supported',
                               fragment=BOOST_ASIO_HAS_LOCAL_SOCKETS_CHECK,
                               use='BOOST', mandatory=False)