diff --git a/extensions/GNUmakefile.in b/extensions/GNUmakefile.in
index 04b4071..2c92698 100644
--- a/extensions/GNUmakefile.in
+++ b/extensions/GNUmakefile.in
@@ -25,8 +25,9 @@ CFLAGS          := @CFLAGS@
 LDFLAGS         := @LDFLAGS@
 regular_CFLAGS  := @regular_CFLAGS@
 kinclude_CFLAGS := @kinclude_CFLAGS@
+xtables_CFLAGS  := @xtables_CFLAGS@
 
-AM_CFLAGS      := ${regular_CFLAGS} -I${top_srcdir}/include ${kinclude_CFLAGS}
+AM_CFLAGS      := ${regular_CFLAGS} -I${top_srcdir}/include ${xtables_CFLAGS} ${kinclude_CFLAGS}
 AM_DEPFLAGS     = -Wp,-MMD,$(@D)/.$(@F).d,-MT,$@
 
 ifeq (${V},)