* lisp/setup.in: New file.

* do/make.el <top-level>: Change destination and source
from setup-PACKAGE.el and setup.template to lisp/setup.el
and lisp/setup.in, respectively.
* do/install (do-the-install!): Don't install setup-PACKAGE.el;
instead, install lisp/setup.el alongside core.el and autoloads.el.
* Makefile.in (all): Add ‘lispdir-symlink’ as dependency.
(clean): Don't delete *~ and setup-PACKAGE.el.;
instead, delete lisp/setup.el.
(maintainer-clean): Also delete lisp/$(PACKAGE).
(lisp/$(PACKAGE), lispdir-symlink): New targets.
* README: Split first section into Installation and Usage;
mention optionality of installation in Installation;
update usage info, including usage from build tree, in Usage.
* setup.template: Delete file.