emacs install: make sure all components to be installed are there

`make install-emacs` will copy $(emacs_sources), $(emacs_images) and
$(emacs_bytecode) to their target directories. $(emacs_bytecode) was
already a prerequisite of make install-emacs as these obviously needed
to be build. Until a while ago all of $(emacs_sources) was available
in the repository, but now it includes `notmuch-version.el` which
is generated. In the future we may have generated emacs images too.