diff --git a/src/Makefile b/src/Makefile
index b42f945..c703321 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -57,7 +57,7 @@ endif
 VERSION = 0.4.2
 VERSION_SCRIPT  := libbpftune.map
 
-CFLAGS = -fPIC -Wall -Wextra -g -I../include -std=c99
+CFLAGS += -fPIC -Wall -Wextra -g -I../include -std=c99
 
 CFLAGS += -DBPFTUNE_VERSION='"$(BPFTUNE_VERSION)"' \
 	  -DBPFTUNER_PREFIX_DIR='"$(prefix)"' \