Consistency with libtool wrt compatibility version and version

On OS X, SOVERSION and VERSION properties impact

• the filenames and symbolic links
• the compatibility version and version properties of the generated libraries.

On the other hand, libtool generates different values of compatibility version and version than cmake. These values are are defined based on a version info of the form current:revision:age. libtool's versioning is a bit weird, but the rule is:

• ABI version is current - age
• soversion is ABI
• compatibility version is current
• current version is compatibility.revision.0

In order for cmake to be able to generate the same result as libtool, we probably need to allow the compatibility version and version properties of the dylib files to be generated independently of SOVERSION and VERSION, or to allow passing a libtool version info property to targets.

Linking to zeromq issue https://github.com/zeromq/libzmq/issues/2887.