Add GemRB version number to gemrb_core library.

This adds the GemRB version number and major version number to the gemrb_core library properties, as enforced by Debian packaging guidelines for all shared libraries.
Wouldn't his result in "v0" until we hit 1.x? I think you could just hardcode it to a dummy number, as nobody else is dependant on the version/api. Usually soversions are [0-9.], but I don't know if any tools care.