systemd: install service file using `systemdsystemunit_DATA`

When there is an autoconf variable `somethingdir`, we can use
`something_DATA` in automake to install things to this directory. This
is more convenient than using local targets We still have to protect
the use of those variables with conditional macros since they can be
empty or set to no. There is also a drawback about distcheck trying to
set prefix and not expecting things to be installed outside of prefix.

Do the same thing for OS X launchd daemon configuration file.