Allow to remove systemd library

Allow to remove systemd library

This change allows to compile a binder
that doesn't use libsystemd.

The restriction implied is that the neither
the systemd event loop nor the systemd dbus
facilities are availables. Consequently,
the dbus transparancy of APIs isn't available.

Also the work isn't fully complete because
afb-ws-client doesn't handle the removal of
libsystemd.

A more general work still have to be made to
allow the binder to run as library using an
external event loop.

Change-Id: I27a80862868b9c5a0453011a1d8aa3ce75cb5f6e
Signed-off-by: Jose Bollo <jose.bollo@iot.bzh>