#64 Re-introduce soname versioning

[Volker Fröhlich]

With CMake it's as easy as adding:

set_target_properties (gpstk PROPERTIES VERSION 23.0.0 SOVERSION 23)

Please try to stick with the conventions outlined here: http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html and http://plan99.net/~mike/writing-shared-libraries.html

Also consider to register the library on http://upstream-tracker.org/ to monitor for incompatible changes comfortably.