another Makefile fix

[Henry Cejtin]

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:

    if [ -z "$CVSROOT" ]; then                      \
            echo "you must set up CVS" >&2          \
            exit 1                                  \
    else                                            \
            ...                                     \
    fi