Makefile: hardcode systemd unit installation path

The path can't be relative to PREFIX.
This is how things work with systemd...

Sources:
- systemd/systemd#19414
- https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html

Makefile

@@ -13,7 +13,8 @@ BINDIR ?= $(PREFIX)/bin
 MANDIR ?= $(PREFIX)/share/man
 MAN1DIR ?= $(MANDIR)/man1
 SYSCONFDIR ?= $(PREFIX)/etc
-SYSTEMDUNITDIR ?= $(SYSCONFDIR)/systemd/system
+# Systemd hardcodes its unit path list
+SYSTEMDUNITDIR ?= /etc/systemd/system
 PANDOC := $(shell command -v pandoc 2> /dev/null)
 
 .PHONY: all clean install uninstall format test