liblzma: Use symbol versioning.

Symbol versioning is enabled by default on GNU/Linux,
other GNU-based systems, and FreeBSD.

I'm not sure how stable this is, so it may need
backward-incompatible changes before the next release.

The idea is that alpha and beta symbols are considered
unstable and require recompiling the applications that
use those symbols. Once a symbol is stable, it may get
extended with new features in ways that don't break
compatibility with older ABI & API.

The mydist target runs validate_map.sh which should
catch some probable problems in liblzma.map. Otherwise
I would forget to update the map file for new releases.

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