Build fswatch using the GNU Build System

Hi,

following up [this][brew] conversation about the Brew recipe of `fswatch` and some work I independently made on a [similar program][fsw], I decided to dedicate some time to reimplement some of the things I made in `fsw`.

If you pull this request into the repository, you'll be able to build `fswatch` with the GNU Build System and the typical chain of commands:

    ./configure && make && make install

To build a release tarball (which you can then add to every GitHub release of `fswatch`), the following command can be used:

    make distcheck

Cheers,
-- Enrico

[brew]: https://github.com/Homebrew/homebrew/pull/13110#issuecomment-40681687
[fsw]: https://github.com/emcrisostomo/fsw