Use a proper makefile rather than the `setup' script.

Use a proper makefile rather than the `setup' script.

And there was rejoicing.  Surprisingly, the makefile is slightly shorter
than the script used to be, but all of the functionality is there.

Things are, unsurprisingly, organized a little differently.  I can
gather definitions together thematically, as opposed to grouping
according to the technicalities of installation.  The path hacks --
well, only the ssh one, because there aren't any others -- have moved
under bin/ now, because that means I can use the same machinery as the
other script links.

The makefile is also easier to tweak its behaviour to cope with strange
situations.  So that's nice.