Fix gen-autoloads make target

The package autoload is deprecated since Emacs 29.1. But we want to switch to
Eask anyway.

Makefile

@@ -1,4 +1,6 @@
-EMACS=emacs
+EMACS = emacs
+BATCH = $(EMACS) -Q -batch -L .
+
 export LC_ALL=C
 
 all: clean compile gen-autoloads test
@@ -8,10 +10,14 @@ clean:
 	git clean -Xf
 
 compile:
-	${EMACS} -Q -batch -L . -f batch-byte-compile tests/org-journal-test.el org-journal.el
+	$(BATCH) -f batch-byte-compile tests/org-journal-test.el org-journal.el
 
 gen-autoloads:
-	${EMACS} -Q -batch -L . -l org-journal.el --eval '`(generate-file-autoloads "org-journal-autoloads.el")`'
+	$(BATCH) \
+		--eval "(require 'autoload)" \
+		--eval '(setq make-backup-files nil)' \
+		--eval "(setq generated-autoload-file (concat command-line-default-directory \"/\" \"org-journal-autoloads.el\"))" \
+		-f batch-update-autoloads "."
 
 test:
-	${EMACS} -Q -batch -L . -l tests/org-journal-test -f ert-run-tests-batch-and-exit
+	$(BATCH) -l tests/org-journal-test -f ert-run-tests-batch-and-exit