From https://github.com/Hummer12007/brightnessctl/issues/67

--- a/Makefile
+++ b/Makefile
@@ -18,6 +18,8 @@ MODE = ${MODE_${INSTALL_UDEV_RULES}}
 ifdef ENABLE_SYSTEMD
 	CFLAGS += ${shell pkg-config --cflags libsystemd}
 	LDLIBS += ${shell pkg-config --libs libsystemd}
+	CFLAGS += ${shell pkg-config --cflags libelogind}
+	LDLIBS += ${shell pkg-config --libs libelogind}
 	CPPFLAGS += -DENABLE_SYSTEMD
 	INSTALL_UDEV_RULES=0
 	MODE = 0755
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