To build Sorts Mill fonts from Mercurial or Git sources, first go into
the 'tools' directory and install the Sorts Mill Tools, using
something like

    autoreconf --install
    make
    make install

Then, if the desired fonts are far enough along in development, go
into the appropriate directory and once again do something like

    autoreconf --install
    make
    make install

If the desired fonts are not that far along, then I don't know what
you should do. :)