Avoid having a directory as a prerequisite in doc Makefiles.
It does not play well with time-stamping.
* doc/misc/Makefile.in ($(infodir)): Delete rule.
(mkinfodir): New. Use it in all the info rules, rather than depending
on infodir.
* doc/lispref/Makefile.in ($(infodir)): Delete rule.
(mkinfodir): New.
($(infodir)/elisp): Use $mkinfodir instead of infodir.
* doc/lispintro/Makefile.in ($(infodir)): Delete rule.
(mkinfodir): New.
($(infodir)/eintr): Use $mkinfodir instead of infodir.
* doc/emacs/Makefile.in ($(infodir)): Delete rule.
(mkinfodir): New.
($(infodir)/emacs): Use $mkinfodir instead of infodir.