Avoid having a directory as a prerequisite in doc Makefiles.

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.