Don't require socks_mosq.c in cpp library.

lib/cpp/mosquittopp.cpp

@@ -281,7 +281,11 @@ void mosquittopp::user_data_set(void *userdata)
 
 int mosquittopp::socks5_set(const char *host, int port, const char *username, const char *password)
 {
+#ifdef WITH_SOCKS
 	return mosquitto_socks5_set(m_mosq, host, port, username, password);
+#else
+	return MOSQ_ERR_NOT_SUPPORTED;
+#endif
 }