diff --git a/src/Makefile.inc b/src/Makefile.inc
index cbdf1f174..b274c1d4a 100644
--- a/src/Makefile.inc
+++ b/src/Makefile.inc
@@ -1,6 +1,6 @@
 CC = gcc
 CXX = g++
-CFLAGS = -Wall -W -O2
+CFLAGS += -Wall -W -O2
 CXXFLAGS = $(CFLAGS)
 
 all: all-local all-recursive