This pulls in https://github.com/kwilczynski/ruby-magic/pull/5, which
brings significant improvements in the building of `libmagic`:

- Use `mini_portile2` gem to build the C extension.
- Clean up unneeded files after build.
- By default, build a static library of `libmagic` and link it.
- Support disabling of static library generation via the
  `--disable-static` argument.
- Support for using the system library via the `--use-system-libraries`
  argument.