Delete all three Tupfiles

This is the first step in transitioning from Tup to GNU Make.  The
reason for the change is that we run into problems when using LNVL as
a sub-module for our game, which also uses Tup.  Building the game
executes each Tupfile in LNVL, which is the correct behavior for Tup,
but that causes errors and failures to create files.

In all honesty this very likely reflects an error and misunderstanding
on my part, and not a problem with Tup, as it has been a pleasure to
use with other projects.  But I need to get things done and cannot
afford to spend much time tracing down mistakes in Tupfiles.  Make may
be old but it’s reliable and will be easier to control for building our
game, combining resources, etc.
Remove all mention of Tup from the README

Later we will replace this with instructions on using GNU Make to
build the library.
Add a Makefile to replace the deleted Tupfiles

This patch adds a GNU Makefile to the top-level directory which
can perform five tasks:

1. Generating HTML documentation from the Markdown sources by using
the now ‘docs/build-html.sh’ shell script.

2. Create a TAGS file for GNU Emacs.

3. Create the ‘src/settings.lua’ file from the example file that the
project provides.

4. Zip all of the necessary source files into a LÖVE archive.

5. Delete all of the files that the tasks above can create in order to
have a clean slate.

The commentary in the Makefile describes the necessary software that
some of its rules depend on and where to find them, along with a
description of the behavior of running ‘make all’, the most common
use-case.
Document the use of GNU Make in the README
Merge branch 'switch-from-tup-to-make'

* switch-from-tup-to-make:
  Document the use of GNU Make in the README
  Add a Makefile to replace the deleted Tupfiles
  Remove all mention of Tup from the README
  Delete all three Tupfiles