Minor doc Makefile.in clean-up.

Minor doc Makefile.in clean-up.

* doc/emacs/Makefile.in (SHELL): Set it.
(INFO_TARGETS, DVI_TARGETS): Remove variables.
(info, dvi): Replace above variables with their expansions.
(info): Move mkdir from here...
($(infodir)/emacs): ... to here (for parallel builds).
(pdf): New target.
($(infodir)/emacs): Pass -o option to makeinfo.
(.PHONY): Declare clean rules.
(maintainer-clean): Delete dvi and pdf files.
Guard against cd failures.  Use a more restrictive delete.

* doc/lispref/Makefile.in (dvi, pdf, $(infodir)): New targets.
($(infodir)/elisp): Ensure target directory exists.  Use $@.
(clean): No 'make.out' or 'core' files.
(.PHONY): Declare clean rules.
(maintainer-clean): Delete pdf file.  Guard against cd failures.