meson only supports the top level (no subdirs), so this makes the transition
easier.
For this to work with autotools, we need to *disable* the subdir-objects option,
that autotools wants us to enable for forward-compatibility.
This results in a bunch of warnings at autoreconf-time, but we don’t care, given
that we intend to switch away from autotools. Both build systems working next to
each other (as best as they can) is more important.
related to #4086
ef8935b1db