FIX: The systemd service file is always installed

Makefile

@@ -67,7 +67,7 @@ install_scripts: ## Install scipts
 install_scripts: scripts
 	install -Dm755 scripts/beesd $(DESTDIR)$(PREFIX)/sbin/beesd
 	install -Dm644 scripts/beesd.conf.sample $(DESTDIR)/$(ETC_PREFIX)/bees/beesd.conf.sample
-ifneq (SYSTEMD_SYSTEM_UNIT_DIR,)
+ifneq ($(SYSTEMD_SYSTEM_UNIT_DIR),)
 	install -Dm644 scripts/beesd@.service $(DESTDIR)$(SYSTEMD_SYSTEM_UNIT_DIR)/beesd@.service
 endif