I also got a warning after the installation BUT everything does appear to be working reguardless.
root@localhost /opt/alarm-fake-hwclock # systemctl enable /opt/alarm-fake-hwclock/systemd/fake-hwclock.service
Created symlink from /etc/systemd/system/local-fs-pre.target.wants/fake-hwclock.service to /opt/alarm-fake-hwclock/systemd/fake-hwclock.service.
Created symlink from /etc/systemd/system/fake-hwclock.service to /opt/alarm-fake-hwclock/systemd/fake-hwclock.service.
The unit files have no [Install] section. They are not meant to be enabled
using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
.wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
D-Bus, udev, scripted systemctl call, ...).
Installed this on a RPi running CentOS 7 ARM.
It appears to be working but I had to do things slightly differently than your documentation says:
I had to run the systemctl command with the full path to the .service file
/opt/alarm-fake-hwclock # systemctl enable /opt/alarm-fake-hwclock/systemd/fake-hwclock.service
I also got a warning after the installation BUT everything does appear to be working reguardless.