diff --git a/lib/libunbound/Makefile b/lib/libunbound/Makefile
index e348c665041a..76a61f6210a3 100644
--- a/lib/libunbound/Makefile
+++ b/lib/libunbound/Makefile
@@ -10,7 +10,7 @@ UNBOUNDDIR= ${.CURDIR}/../../contrib/unbound
 LIB=	unbound
 PRIVATELIB=
 
-CFLAGS= -I${UNBOUNDDIR} -I${LDNSDIR} -I${.OBJDIR}
+CFLAGS+= -I${UNBOUNDDIR} -I${LDNSDIR} -I${.OBJDIR}
 
 SRCS=	alloc.c as112.c autotrust.c config_file.c configlexer.l configparser.y \
 	context.c dname.c dns.c dns64.c dnstree.c fptr_wlist.c infra.c \
diff --git a/usr.sbin/unbound/anchor/Makefile b/usr.sbin/unbound/anchor/Makefile
index 64e01d2ef836..d2431eb6d19a 100644
--- a/usr.sbin/unbound/anchor/Makefile
+++ b/usr.sbin/unbound/anchor/Makefile
@@ -9,7 +9,7 @@ EXPATDIR= ${.CURDIR}/../../../contrib/expat
 
 PROG=	unbound-anchor
 SRCS=	unbound-anchor.c
-CFLAGS=	-I${UNBOUNDDIR} -I${LDNSDIR} -I${EXPATDIR}/lib
+CFLAGS+=	-I${UNBOUNDDIR} -I${LDNSDIR} -I${EXPATDIR}/lib
 LIBADD=	unbound bsdxml ssl crypto pthread
 MAN=	unbound-anchor.8
 
diff --git a/usr.sbin/unbound/checkconf/Makefile b/usr.sbin/unbound/checkconf/Makefile
index 884465ba70a0..98ac3bb6f856 100644
--- a/usr.sbin/unbound/checkconf/Makefile
+++ b/usr.sbin/unbound/checkconf/Makefile
@@ -8,7 +8,7 @@ UNBOUNDDIR= ${.CURDIR}/../../../contrib/unbound
 
 PROG=	unbound-checkconf
 SRCS=	unbound-checkconf.c worker_cb.c
-CFLAGS=	-I${UNBOUNDDIR} -I${LDNSDIR}
+CFLAGS+=	-I${UNBOUNDDIR} -I${LDNSDIR}
 LIBADD=	unbound pthread
 MAN=	unbound-checkconf.8
 
diff --git a/usr.sbin/unbound/control/Makefile b/usr.sbin/unbound/control/Makefile
index 824e44111fc0..9aab04a4c43d 100644
--- a/usr.sbin/unbound/control/Makefile
+++ b/usr.sbin/unbound/control/Makefile
@@ -8,7 +8,7 @@ UNBOUNDDIR= ${.CURDIR}/../../../contrib/unbound
 
 PROG=	unbound-control
 SRCS=	unbound-control.c worker_cb.c
-CFLAGS=	-I${UNBOUNDDIR} -I${LDNSDIR}
+CFLAGS+=	-I${UNBOUNDDIR} -I${LDNSDIR}
 LIBADD=	unbound crypto ssl pthread
 MAN=	unbound-control.8
 
diff --git a/usr.sbin/unbound/daemon/Makefile b/usr.sbin/unbound/daemon/Makefile
index f90e06ee17cd..af42975b10eb 100644
--- a/usr.sbin/unbound/daemon/Makefile
+++ b/usr.sbin/unbound/daemon/Makefile
@@ -8,7 +8,7 @@ UNBOUNDDIR= ${.CURDIR}/../../../contrib/unbound
 
 PROG=	unbound
 SRCS=	acl_list.c cachedump.c daemon.c remote.c stats.c unbound.c worker.c
-CFLAGS=	-I${UNBOUNDDIR} -I${LDNSDIR}
+CFLAGS+=	-I${UNBOUNDDIR} -I${LDNSDIR}
 LIBADD=	unbound util ssl crypto pthread
 MAN=	unbound.8 unbound.conf.5