Fix support for older libwebsocket versions

LWS_WITH_EXTERNAL_POLL a new optional define for libwebsocket 4.x.
Earlier versions always supported it, without the compile-time define.

This fixes support for libwesocket 2.x (Debian, Ubuntu), and 3.x.
Also for Windows, LWS_WITH_GLIB is not available.

libs/surfaces/websockets/server.cc

@@ -116,13 +116,6 @@ WebsocketsServer::WebsocketsServer (ArdourSurface::ArdourWebsockets& surface)
 int
 WebsocketsServer::start ()
 {
-#if !defined(LWS_WITH_GLIB) && !defined(LWS_WITH_EXTERNAL_POLL)
-	PBD::error << "ArdourWebsockets: check your libwebsockets was compiled"
-	              " with LWS_WITH_GLIB or LWS_WITH_EXTERNAL_POLL enabled"
-	           << endmsg;
-	return -1;
-#endif
-
 #ifndef NDEBUG
 	lws_set_log_level (LLL_ERR | LLL_WARN | LLL_DEBUG, 0);
 #endif
@@ -145,6 +138,17 @@ WebsocketsServer::start ()
 		return -1;
 	}
 
+#ifndef LWS_WITH_GLIB
+	/* sometimes LWS_WITH_EXTERNAL_POLL is missing from lws_config.h
+	   but the feature is still available, hence this runtime check */
+	if (_fd_ctx.empty ()) {
+		PBD::error << "ArdourWebsockets: check your libwebsockets was compiled"
+		              " with LWS_WITH_GLIB or LWS_WITH_EXTERNAL_POLL enabled"
+		           << endmsg;
+		return -1;
+	}
+#endif
+
 	return 0;
 }