cmake: Use -fvisibility=hidden on Unix if supported

This avoids accidentally exporting all of our private symbols as part of the LIBDBUS_PRIVATE_x.y.z verdef, including ones that don't need to be visible outside the shared library even when testing internals (_dbus_watch_set_handler is a good example).

Fixing this for cmake as well helps with checks for differences between build systems. This bug was only discovered because the test case added with !379 (merged) was also added to autotools.

Resolves: #437