13.2 BETA2: how do debug META_MODE?

From: Peter <pmc_at_citylink.dinoex.sub.org>
Date: Tue, 21 Feb 2023 03:45:12 UTC
Hi folks,

  on /some/ of my nodes, META_MODE seems not being honored anymore:
I had to build them another time, and the lengthy lib/clang gets
built all over again (tried two times).
This is so since 13.2 (BETA2). It did work in 13.1 (RELENG), at least
according to the timing from the logfiles. 

Now I'm trying to figure out the difference, because I have some
nodes where it appears to more-or-less work (have seen buildworld
take 5 minutes), and others where it doesn't (take an hour to build).
The thing is scripted, so it is not so very likely an operator error
(while not impossible either).

But it seems difficult to figure out details: "make -n" seems to not
care about META_MODE, while META_MODE suppresses all useful output from
make. And the docs say there are *.meta files (yes there are), but no
info about how to verify their content, or how to get make tell what
it is going to do and why (and the buildworld is not the most easy
to understand target)...

So, some inspiration would be welcome...