Improve Makefile...

* `make` now just pre-processes the `.nanorc` files and places the output
  in the `build/` directory. This is to allow the files to be inspected
  or copied around without the need to install anything.
* `make install` now installs both the concatenated `ALL.nanorc` bundle and
  each individual language under `~/.nano/syntax/`, and expects users to
  manually `include` the languages they wish to use from their `~/.nanorc`.
* `sudo make install-global` does the same as above, but under the
  `/usr/local/share/nano/` directory, to allow the files to be included
  from anywhere (including `/etc/nanorc`).