Add SONAME versioning

[robert-scheck]

A simple attempt to address #11. If you would like to go into this direction in general, it could be improved, e.g. moving the soversion definitions to configure.in and/or making the shared linking for the binaries optional using a ./configure option.

This modernizes my previous pull request from https://bitbucket.org/ripencc/bgpdump/pull-requests/28/introduce-soname-versioning proposed at 2019-04-28.