another Makefile fix

[Matthew Fluet]

> Here  is  another change that should be done to the relevant Makefile's (just
> doc/web/Makefile at the moment, although bin/make-rpms  should  probably  get
> this to): Test if CVSROOT is in the environment and fail, with an appropriate
> complaint if it isn't.  Something like this:

Maybe, although I never set CVSROOT, because mlton is in one repository
and at various other times, I've used other CVS repositories (e.g.,
a local one for class projects).  So, I always do cvs -d... co ... and
then the CVSROOT is kept in the corresponding CVS directories and I never
need to enter it again.