More flexible make install paths

[tleydxdy]

Distribution (run cat /etc/os-release):
Arch Linux

Issue/Bug Description:
When building the package for Arch, I found out that Arch want the dbus configuration in /usr/share/dbus-1/system.d/system76-power.conf and the systemd service file in /usr/lib/systemd/system/system76-power.service however the current makefile can't be made to does that https://github.com/pop-os/system76-power/blob/master/Makefile#L38

So I wonder if this can be accommodated somehow. I'm not sure what's the difference between /etc/dbus-1 and /usr/share/dbus-1 on debian-famliy

Thanks.

[crawfxrd]

Looking at every other systemd service file on a system, it should be installed to $(libdir) and a symlink added in $(sysconfdir).

[tleydxdy]

Per my understanding /etc should be where user added/edited config goes, and /usr/lib and /usr/share are where package installed configs should go. I do see that pop-os seems to do things differently for some packages but not all, I don't know why.