The path conversion is done automatically if msys' builtin
shell commands are used.
R=rsc1, peterGo, brainman, Mr_Dark, r
CC=golang-dev
https://golang.org/cl/4452042
Otherwise if there is an installed /usr/bin/6g the build
will use that one instead of the one it builds.
R=r
CC=golang-dev
https://golang.org/cl/4249045
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
