Fix make distcheck if systemd is installed

Fix make distcheck if systemd is installed

The automatically determined path for systemd units is an absolute system
path that doesn't respect $(prefix).  That's a problem for make distcheck,
which is usually ran as regular user and it's not expected to have any
impact on the system (it does a local install in a subdir).  To avoid
these issues we override the configure flags used by make distcheck and
set the path to one relative to the specified prefix.