new configure option --with-init-dir=PATH

$ ./configure --with-init-style=redhat-systemd
.....
    Paths:
         Netatalk lockfile:       /var/lock/netatalk
         init directory:          /lib/systemd/system

$ ./configure --with-init-style=redhat-systemd --with-init-dir=/foo/bar
.....
    Paths:
         Netatalk lockfile:       /var/lock/netatalk
         init directory:          /foo/bar