Makefile: remove includes for maintainer targets, these will go
into local.mk on the server
default.mk: introduce $(datadir) to replace $(etcdir) in etc/Makefile
doc/Makefile: replace $etcdir) with $(datadir)
lisp/Makefile: record ODT styles location as defconst in org-install
{doc,etc,lisp}/Makefile: prefix all installation targets with $(DESTDIR)