emacs-build-system installs too many files

[Arun Isaac <arunisaac@systemreboot.net>]

The install phase of the emacs-build-system simply copies all files to
the output directory. This ends up copying many unnecessary files such
as README, Makefile, tests, hidden files such as .gitignore,
.travis.yml, etc. MELPA package recipes, on the other hand, copy only a
subset of the files. This is provided by the :files property in their
recipe format (https://github.com/melpa/melpa#recipe-format). Quoting
from their github page:

#+BEGIN_QUOTE

:files is an optional property specifying the elisp and info files used
to build the package. Please do not override this if the default value
(below) is adequate, which it should usually be:

  ("*.el" "*.el.in" "dir"
   "*.info" "*.texi" "*.texinfo"
   "doc/dir" "doc/*.info" "doc/*.texi" "doc/*.texinfo"
   (:exclude ".dir-locals.el" "test.el" "tests.el" "*-test.el" "*-tests.el"))

This option is necessary when there are multiple packages in the
repository and thus the package should only be built from a subset of
.el files. For example, elisp test files should not normally be
packaged. Any file specified at any path in the repository is copied to
the root of the package.

...

Note that elisp in subdirectories is never included by default, so you
might find it convenient to separate auxiliiary files such as tests into
subdirectories to keep packaging simple.

#+END_QUOTE

I suggest we modify the emacs-build-system to include a #:files argument
similar to MELPA's :files property. If others are fine with this, I will
start working on a patch.

Regards,
Arun Isaac.