build: work with GNU Make 3.82
build: work with GNU Make 3.82

Fix building on GNU Make 3.82 (caused by mixed implicit and normal
rules).

The issue was introduced in changeset 6110:ca0beac3b543.

R=rsc
CC=golang-dev
http://codereview.appspot.com/2100044

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go