Give compile time warning for libwebsockets

If it is compiled without external poll support.

Closes #2060. Thanks to Yannic Schröder.

ChangeLog.txt

@@ -5,6 +5,8 @@ Broker:
   because plugins should never be allocating their own instance of this
   struct, and currently none of the struct members are used for anything, so a
   plugin should not be accessing them.
+- Give compile time warning if libwebsockets compiled without external poll
+  support. Closes #2060.
 
 Clients:
 - Fix possible loss of data in `mosquitto_pub -l` when sending multiple long

src/mosquitto_broker_internal.h

@@ -25,6 +25,9 @@ Contributors:
 
 #ifdef WITH_WEBSOCKETS
 #  include <libwebsockets.h>
+#  if LWS_LIBRARY_VERSION_NUMBER >= 3002000 && !defined(LWS_WITH_EXTERNAL_POLL)
+#    warning "libwebsockets is not compiled with LWS_WITH_EXTERNAL_POLL support. Websocket performance will be unusable."
+#  endif
 #endif
 
 #include "mosquitto_internal.h"