build script tweaks

build script tweaks

factor out environment variable checks.
infer $GOROOT etc during build if not set.
it's still necessary to set them for yourself
to use the standard Makefiles.

when running all.bash, don't recompile all the
go packages in run.bash, since make.bash already did.

R=r
CC=golang-dev
https://golang.org/cl/609042