Only reenter the main loop if AT_SPI_CLIENT is set

Only reenter the main loop if AT_SPI_CLIENT is set

If AT_SPI_CLIENT is not set, call dbus_connection_read_write_dispatch instead
of re-entering the main loop.  Fixes a deadlock with threaded gtk
applications (ie, gnome-do) where gdk would lock while sending a signal,
at-spi would re-enter the main loop, and gdk would try to process another
signal an re-acquire the lock, causing deadlock because the lock is not
reentrant.