OSX_COMPATIBILITY_VERSION
-------------------------

What current version number is this target for OSX.

For shared libraries on Mach-O systems (e.g. macOS, iOS)
the ``OSX_COMPATIBILITY_VERSION`` property correspond to
``compatibility version`` and :prop_tgt:`OSX_CURRENT_VERSION` to
``current version``.
See the :prop_tgt:`FRAMEWORK` target property for an example.

Versions of Mach-O binaries may be checked with the ``otool -L <binary>``
command.  If ``OSX_COMPATIBILITY_VERSION`` is not set, the value of
the :prop_tgt:``SOVERSION`` property will be used.
