Nice patch Antoine!  The method of partitioning the object sets into frozen and
non-frozen is nice.

> The proposed "-X" or BOOTSTRAP_PY approaches would be difficult, since there
> doesn't appear to be an easy way to generate a given file (the Python
> interpreter) twice in a Makefile dependency chain: the first time using the
> stale importlib, the second with the fresh importlib.

I experimented with the env var approach and came to the exact same conclusion
-- you would have to build the interpreter twice.

This patch looks good to me (I did have to rebase it against trunk, though;
it doesn't apply cleanly right now).  I did some basic regression testing and ran
it through the original scenario from issue12370 that got us thinking this problem.
I saw no issues.  Nice work.