diff -Naurd openl2tp-1.8.orig/Makefile openl2tp-1.8/Makefile
--- openl2tp-1.8.orig/Makefile	2018-07-03 04:59:51.000000000 +0900
+++ openl2tp-1.8/Makefile	2018-07-03 05:00:44.133460163 +0900
@@ -155,24 +155,23 @@
 endif
 
 CPPFLAGS=		$(CPPFLAGS.l2tptest) $(CPPFLAGS-y)
-CFLAGS=			-I. -Iusl -Icli -isystem include \
+CFLAGS+=			-I. -Iusl -Icli -isystem include \
 				-MMD -Wall -Wno-strict-aliasing \
 				$(CPPFLAGS) $(CPPFLAGS.dmalloc) \
 				-DSYS_LIBDIR=$(SYS_LIBDIR)
 LDFLAGS.l2tpd=		-Wl,-E -L. -Lusl -lusl -ldl $(LIBS.dmalloc) -lc
 LDFLAGS.l2tpconfig=	-Lcli -lcli -lreadline $(LIBS.dmalloc) $(READLINE_LDFLAGS) -lc
 
-OPT_CFLAGS?=		-O
-
 ifeq ($(L2TP_DEBUG),y)
-CFLAGS.optimize=	-g
-CFLAGS.optimize+=	-DDEBUG
+CFLAGS.opt=	-g
+CFLAGS.opt+=	-DDEBUG
 else
-CFLAGS.optimize=	$(OPT_CFLAGS)
+CFLAGS.opt=	$(OPT_CFLAGS)
 endif
+CFLAGS.optimize= $(CFLAGS) $(CFLAGS.opt)
 export CFLAGS.optimize
 
-CFLAGS+=		$(CFLAGS.optimize)
+CFLAGS+=		$(CFLAGS.opt)
 
 ifeq ($(L2TP_USE_ASYNC_RPC),y)
 CPPFLAGS+=		-DL2TP_ASYNC_RPC