define _OPENBSD_SOURCE for reallocarray on NetBSD
Report by and debugged with Hisashi T Fujinaka, dtucker nailed
the problem (lack of prototype causing return type confusion).
diff --git a/configure.ac b/configure.ac
index e64386f..373d21b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -839,6 +839,7 @@
if test "x$withval" != "xno" ; then
need_dash_r=1
fi
+ CPPFLAGS="$CPPFLAGS -D_OPENBSD_SOURCE"
AC_DEFINE([SSH_TUN_FREEBSD], [1], [Open tunnel devices the FreeBSD way])
AC_CHECK_HEADER([net/if_tap.h], ,
AC_DEFINE([SSH_TUN_NO_L2], [1], [No layer 2 tunnel support]))
