Add a SOVERSION for proper versioning of the libnordlicht ABI

libnordlicht should have a clean versioning of the binary interface so it can be
properly linked to other applications. The SOVERSION property can simply be
increased if the ABI changes, and CMake takes care of all the rest.

I recommend reading [this FAQ](http://www.faqs.org/docs/Linux-HOWTO/Program-Library-HOWTO.html#AEN135)
for a list of cases when the ABI version should (must?) be increased.