diff --git a/gtp/Makefile.am b/gtp/Makefile.am
index b154ee1..ea73a67 100644
--- a/gtp/Makefile.am
+++ b/gtp/Makefile.am
@@ -2,7 +2,7 @@
 
 include_HEADERS = gtp.h pdp.h
 
-CFLAGS = -O2 -fno-builtin -Wall -DSBINDIR='"$(sbindir)"' -ggdb
+AM_CFLAGS = -O2 -fno-builtin -Wall -DSBINDIR='"$(sbindir)"' -ggdb
 
 libgtp_la_SOURCES = gtp.c gtp.h gtpie.c gtpie.h pdp.c pdp.h lookupa.c lookupa.h queue.c queue.h