Write updated loaddefs to a temporary file and rename into place.

In a parallel build, byte compilation can be running at the same times
as loaddefs.el is being regenerated.  However, in a CANNOT_DUMP build,
loaddefs.el is read at startup and must always be in a usable state.

* lisp/Makefile.in ($(lisp)/loaddefs.el): Write generated output to
loaddefs.el.new and then rename it to loaddefs.el.

lisp/Makefile.in

@@ -179,14 +179,18 @@ $(lisp)/finder-inf.el:
 # Note that we set no-update-autoloads in _generated_ leim files.
 # If you want to allow autoloads in such files, remove that,
 # and make this depend on leim.
+#
+# Write to a temporary file in case we're doing a parallel build and a
+# CANNOT_DUMP-mode Emacs needs to read loaddefs at startup.
 autoloads .PHONY: $(lisp)/loaddefs.el
 $(lisp)/loaddefs.el: $(LOADDEFS)
 	@echo Directories for loaddefs: ${SUBDIRS_ALMOST}
 	$(AM_V_GEN)$(emacs) -l autoload \
 	    --eval '(setq autoload-ensure-writable t)' \
 	    --eval '(setq autoload-builtin-package-versions t)' \
-	    --eval '(setq generated-autoload-file (expand-file-name (unmsys--file-name "$@")))' \
+	    --eval '(setq generated-autoload-file (expand-file-name (unmsys--file-name "$(lisp)/loaddefs.tmp")))' \
 	    -f batch-update-autoloads ${SUBDIRS_ALMOST}
+	mv -f $(lisp)/loaddefs.tmp $@
 
 # autoloads only runs when loaddefs.el is nonexistent, although it
 # generates a number of different files. Provide a force option to enable