build: only print "You need to add foo to PATH" when needed
build: only print "You need to add foo to PATH" when needed

Fixes issue 1223.

R=bradfitz
CC=golang-dev
http://codereview.appspot.com/2701041

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