Added check to see if we are running on Mac OS X

OS X's default `readlink` doesn't support '-m' but if the user has
GNU's `readlink` installed, that will work. Also, changed DU= to check
the user's `$PATH` for `dropbox_uploader.sh`
Added check to see if we are running on Mac OS X

OS X's default `readlink` doesn't support '-m' but if the user has
GNU's `readlink` installed, that will work. Also, changed DU= to check
the user's `$PATH` for `dropbox_uploader.sh`