Fix parallel make of docs

* doc/Makefile: Split multiple target "org.texi orgguide.texi".
* mk/org-fixup.el (org-make-manual, org-make-guide):  New functions.
  (org-make-manuals): Make obsolete.

Reported by Ulrich Müller.  https://list.orgmode.org/uee67gr96@gentoo.org/

doc/Makefile

@@ -27,11 +27,17 @@ guide::		orgguide.texi org-version.inc
 	../mk/guidesplit.pl $@/*
 endif
 
-org.texi orgguide.texi:	org-manual.org org-guide.org
+org.texi:	org-manual.org
 	$(BATCH) 				      \
 	  --eval '(add-to-list `load-path "../lisp")' \
 	  --eval '(load "../mk/org-fixup.el")' 	      \
-	  --eval '(org-make-manuals)'
+	  --eval '(org-make-manual)'
+
+orgguide.texi:	org-guide.org
+	$(BATCH) 				      \
+	  --eval '(add-to-list `load-path "../lisp")' \
+	  --eval '(load "../mk/org-fixup.el")' 	      \
+	  --eval '(org-make-guide)'
 
 org-version.inc:	org.texi
 	@echo "org-version: $(ORGVERSION) ($(GITVERSION))"

mk/org-fixup.el

@@ -27,6 +27,21 @@
 (require 'autoload)
 (require 'org-compat "org-compat.el")
 
+(defun org-make-manual ()
+  "Generate the Texinfo file out of the Org manual."
+  (require 'ox-texinfo)
+  (find-file "../doc/org-manual.org")
+  (org-texinfo-export-to-texinfo))
+
+(defun org-make-guide ()
+  "Generate the Texinfo file out of the Org guide."
+  (require 'ox-texinfo)
+  (find-file "../doc/org-guide.org")
+  (org-texinfo-export-to-texinfo))
+
+(make-obsolete 'org-make-manuals
+               "use org-make-manual and org-make-guide."
+               "9.6")
 (defun org-make-manuals ()
   "Generate the Texinfo files out of Org manuals."
   (require 'ox-texinfo)