[WIP] Fixed installation process by following GNU coding standards

| Q               | A
| --------------- | ---
| Bug fix?        | yes (some unexpected make behaviours)
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Tests pass?     | yes
| Related tickets |
| License         | GNU GPLv3

In order to come as close as possible to the [GNU coding standards][2].

The python library of this package is private so it does not install under the standard python `site-package` but on [`$(DESTDIR)$(pkglibdir)`][1].

- [x] added a check for `$(DESTDIR)` feature on the `distcheck` target
- [ ] update the doc
- [ ] clean up commit history
- [ ] optionals:
  - [x] maybe added a small configure script [ref][0]
  - [ ] added uninstall feature

[0]: https://github.com/nylen/cyg-apt/blob/431821aa8a3f2d5317424e7eebfbf39cc60eb760/TODO#L9
[1]: https://github.com/alquerci/cyg-apt/blob/e10d29636f227b45fe2d46b995324152034c648a/bootstrap.mk#L112
[2]: https://www.gnu.org/prep/standards/html_node/Makefile-Conventions.html#Makefile-Conventions