Move "racket/src/cs/bootstrap" from the Racket source repository to
this one, because the bootstrapping implementation needs to track the
Chez Scheme source much more closely than the Racket implementation.
Currently, any Racket v7.1 or later works.
Also update "README.md" and "BUILDING" to get all the information
consistent and in sync with revised build options.
original commit: a9e6e99ea414b4625fe9705e4f3cfd62bbf38ae2
