* There are now only two public version identifiers: "number" is
a single integer that combines Major/minor/release in a single
value of the form Mmmmrrr. This is easy to compare against for
checking feature support. "string" is a displayable text string
of the form "libarchive M.mm.rr".
* The number is present both as a macro (version of the installed header)
and a function (version of the shared library). The string form
is available only as a function.
* Retain the older version definitions for now, but mark them all
as deprecated, to disappear in libarchive 3.0 (whenever that happens).
* Rework the various deprecation conditionals to use ARCHIVE_VERSION_NUMBER.
An ancillary goal is to reduce the number of @...@ substitutions that
are required. Someday, I might even be able to avoid build-time
processing of archive.h entirely.
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