Issue #21166: Prevent possible segfaults and other random failures of
python --generate-posix-vars in pybuilddir.txt build target by ensuring
that pybuilddir.txt is always regenerated when configure is run and
that the newly built skeleton python does not inadvertently import
modules from previously installed instances.
Issue #22199: Make get_makefile_filename() available in Lib/sysconfig.py
for 2.7 to match other versions of sysconfig.
Issue #22199: update Misc/NEWS