mac build: updates to respect UPLOAD_DEST argument to make

mac build: updates to respect UPLOAD_DEST argument to make

the make mac-upload target now requires an UPLOAD_DEST argument to be given,
which is the rsync destination (including trailing '/') to which the version
stamped directory containing the .dmg should be placed.  the account the
build is running as (e.g. 'buildslave') should have ssh access to the account
specified in that dest. one might also consider locking the key down to the
target directory by adding something like
command="rsync --server -vlogDtpr . /home/amduser/public_html/dist/mac-blah/"
to the corresponding authorized_key entry on the target machine.