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When "configure.in" has not changed, autoconf reads the content of
"autom4te", which contains the old version number.

so as to keep the full path unchanged

new goals are associated to old goals directly in the order they appear.
they are all marked obsolete, unless the theory itself is found non
obsolete (thanks to the new checksums for theories)

in other words, reloading a session on a file that did not change results
in non-obsolete goals, even if checksums and shapes are absent (e.g. if
the file was not put under version control)

and merge the why3 and why3session libraries back into one.