WIN32: add versioninfo, and export symbols

WIN32: add versioninfo, and export symbols

* dbus/versioninfo.rc.in: (new file) copied and modified from libgpg-error
 * configure.in: enable libtool's W32 support, extract version information
    for use in versioninfo.rc.in, define DBUS_WIN, define DBUS_UNIX only on
    unix (adapted from libgpg-error)
 * dbus/Makefile.am: switch sources depending on platform,
    compile versioninfo.rc, add dbus-1.def (not used since buildsystem creates
    its own .def file apparently)
(cherry picked from commit cfa717ddb7b9258a862d87cd5b3bdbe582ce106c)