Run config.py with various options and store the results in files.
This script also supports the now-removed config.pl.
This is a framework to run non-regression tests on config.py: run it
with the old version, run it with the new version, and compare the
output.
This is deliberately not a functional test suite so that we don't need
to maintain a set of known outputs. When something changes in
config.py (or config.h), run the script before, run it after, and
check manually whether any differences in the output are acceptable.
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