Build fixes

* Don't use complicated and frail gnome-autogen (eg Bug 537772)
  when autoreconf suffices.
* Add prerequisits to configure.ac.
* Modernise to LT_INIT and AS_HELP.
* Don't throw away X_LIBS information from AC_PATH_XTRA.
* Various desktop Makefile rules: $< is not defined in explicit
  rules. (GNU make extension?)
* Make recursively expands variables, so use it rather than AS_AC_EXPAND.
  This also conforms to the GNU coding standards which mandate that a
  user must be able to "make prefix=/somewhere/else install"
* Check installed glib has g_ptr_array_new_with_free_func which we use.
* Xtst test and following failed because glib-2.0 ended up in LIBS
  thanks to pkg-config, hence save_LIBS dance.
* Fix help message as --disable-relocate is the default.
* Look for dbus-daemon rather than fallback to the bindir we are
  installing to, and remove the DBUS_DAEMONDIR define inherited
  from "dbus".
* Make use of bus/Makefile.am, and actually install at-spi-dbus-bus and
  the desktop file.
* Tested...