build: fix cgo with -j2
build: fix cgo with -j2

a b: c expands to multiple rules,
which then run in parallel.

R=r, iant
CC=golang-dev
http://codereview.appspot.com/2007047

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