doc/go1.4.txt: go generate

CC=golang-codereviews
https://codereview.appspot.com/132890043
cmd/go: add go generate
First cut.

Works well enough to support yacc via
        http://codereview.appspot.com/125620044.

LGTM=alex.brainman, rsc
R=rsc, alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/125580044
cmd/yacc: remove Makefile and build expr using go generate
It now serves as an example for go generate as well as for yacc.

NOTE: Depends on go generate, which is not yet checked in.
This is a proof of concept of the approach.
That is http://codereview.appspot.com/125580044.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/125620044