Add aux files `config.{guess,sub}` and `install-sh`

These files are taken from GNU automake 1.13.3

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Refactor & modernize `.cabal` to `cabal-version>=1.10`

This sets a sensible cabal category (i.e. `Numerical`), extends
`extra-tmp-{files,files}` to make this package self-contained, updates
the bug-report URL, and cleans up the `{-# LANGUAGE #-}` pragma usage in
the source code.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>