Merge pull request #357 from ilovezfs/sysconfdir-dev

**use `$(sysconfdir)` rather than "hardwiring" `/etc`**

This update means the build process now uses the directory path `sysconfdir` to determine where to place the configuration file `shairport-sync.conf`. The default value for `sysconfdir` is `/usr/local/etc` which is used in the BSD family, but `/etc` is normally used in Linux, so, to retain the present behaviour of Shairport Sync, you must add an extra parameter to the `./configure... ` command – the parameter you must add is `--sysconfdir=/etc`

This is one component of the updates needed to use `sysconfdir` properly. The others will be part of a subsequent commit. Also, documentation will need to be updated.