Working in "real-life" or with "distcheck" but not both?

[Johan Persson]

As part of the installation of a project I'm working on I want to
install a systemd service file which is created as part of the
configuration process (by autoconf). 

Now I found myself in the situation where I can choose to have this work
either in real life or when I make "make distcheck" but not both. 

My problem is that me trying to install the service in an absolute
system directory as 

$(INSTALL) -m 644 ${systemd_service} ${DESTDIR}/usr/lib/systemd/system 

will not work with distcheck since it requires the use of ${prefix} in
order for the temporary install/check to work. 

However modifying the install using ${prefix} as in 

$(INSTALL) -m 644 ${systemd_service}
${DESTDIR}${prefix}/lib/systemd/system 

will make the distcheck work but in real life it will only work as long
as the ${prefix} == "/usr" but it will not work if the end user changes
the prefix when running configure. 

I guess I could figure out some "if conditions" to do some workarounds
here depending on the value of prefix but it seems "ugly" to me and
there must be a better way to make it work both with distcheck and in
"real life" but having re-read the automake manual nothing obvious pops
out so I suspect I'm missing something obvious here? 

Is someone offering to make a fool out of me pointing out this "obvious
thing" I'm missing :-)