diff --git a/CMakeLists.txt b/CMakeLists.txt
index fe122ecb32fc53a63edbbf12dbdd2cbd1eac0331..b4f6ff988b1692329822f3bc7cbaf1490f1c0c2b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 2.8.7)
 project(macaon2)
 
-add_definitions("-Wall")
+find_package(FLEX)
 
 # for gcc < 5.3
 #add_definitions("-Wall -std=gnu11" )