Keep cmake generated shared dbus-1 library file name in sync with autotools.

Autotools uses a versioned shared library name derived from libtool.
This patch adds a versioned shared library name to cmake builds for all
supported platforms. Binary compatibility for clients build against older
cmake generated binary packages of dbus is provided; on unix like os
with symbolic links and on Windows with a copy of the shared library.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=74117
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>