allow LWS_SOMAXCONN to be defined at configuretime

Default remains at SOMAXCONN, you can force it at configure time
along these lines

./configure CFLAGS="-DLWS_SOMAXCONN=16384"

Signed-off-by: Andy Green <andy.green@linaro.org>
diff --git a/lib/libwebsockets.c b/lib/libwebsockets.c
index 12359ba..8bc91de 100644
--- a/lib/libwebsockets.c
+++ b/lib/libwebsockets.c
@@ -3040,7 +3040,7 @@
 		wsi->mode = LWS_CONNMODE_SERVER_LISTENER;
 		insert_wsi(context, wsi);
 
-		listen(sockfd, SOMAXCONN);
+		listen(sockfd, LWS_SOMAXCONN);
 		lwsl_info(" Listening on port %d\n", port);
 
 		/* list in the internal poll array */