Clarify MacOS buld instructions

    git clone https://git.savannah.gnu.org/git/gforth.git

    cd gforth

    . ./install-deps.sh # install all known dependencies for a full build

    source ./install-deps.sh # install all known dependencies for a full build

    ./BUILD-FROM-SCRATCH

    sudo make install

## Additional info for MacOS ##

You'll get brew and XCode command line tools installed (git will trigger the

latter) if it is not already there.  The install-deps.sh

latter) if it is not already there.  The `install-deps.sh` exports a few

variables, and you probably should put those into your shell setup if you want

to build Gforth latest without sourcing `install-deps.sh` again.

## Alternative: Build from Tarball

If you are building from the tarball, please consult [INSTALL](INSTALL).