The sources include a copy of the header. As of 2016, they are identical
to those that come with tuntap drivers - and future-wise, they should match.
So we use system copy of the header instead, and require the 'header-tun'
package as part of the recipe.

--- ./tunctl.c.~1~	2014-12-14 12:59:41.000000000 +0100
+++ ./tunctl.c	2016-11-20 00:12:57.004129056 +0100
@@ -40,7 +40,7 @@
 #include <strings.h>
 #include <errno.h>
 #include <ctype.h>
-#include "if_tun.h"
+#include <net/if_tun.h>
 
 #ifndef TUNMAXPPA
 #define TUNMAXPPA	20
--- ./Makefile.in.~2~	2014-12-14 12:59:41.000000000 +0100
+++ ./Makefile.in	2016-11-20 00:07:51.307370977 +0100
@@ -31,7 +31,7 @@
 
 all: tunctl
 
-tunctl: $(SRCDIR)/tunctl.c $(SRCDIR)/if_tun.h
+tunctl: $(SRCDIR)/tunctl.c
 	$(CC) $(CFLAGS) $(LDFLAGS) $(SRCDIR)/tunctl.c -o tunctl
 
 install: all