Setup autotools to read .so version info from a separate file

We want to use the same libtool style .so versions in both build systems,
ideally both systems should read the version information from the same
configuration file.

This commit introduces an so.version configuration file and sets up
the autotools to use it.

The version numbers in so.version define the ABI compatibility and should be
updated prior to each release.

implements #323

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