If you run with:

$ export MAKEFLAGS="-j9"
$ make install install-systemd

As in, combine "install" and "install-systemd" in the same command using
parallel make,
the content of .service files could end up in .rules, or otherway around
because GNU
make fill run them in parallel and the commands might ran at the same
time, and both
use same variables $file and same temporary files .install.tmp

This was reported at,
https://bugs.gentoo.org/show_bug.cgi?id=517218

The immediate workaround is,

$ export MAKEFLAGS="-j9"
$ make install
$ make install-systemd

And the attach patch makes it full proof so they can be ran at the same
line.

Thanks,
Samuli