OSX client uses ring-project for its packaging.
when running make-ring --install daemon and client install prefixes are
different. This means ringtones are not packaged properly (they are in
daemon folder /share/ring/ringtones).

This variable can help all clients to point out to a specific external
dir containing ringtones. It will be ignored if not used.

Change-Id: Id22a9c9b1c7df3c11b882c05e693198514ab9458
Tuleap: #652