And do it properly so freebsd and nacl still work.

Change-Id: I6f9f30e93ceae6dee59215ed608c6a158bdbdbb0
Reviewed-on: https://go-review.googlesource.com/14280
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>