bug#4202: 23.0.60; CVS emacs fails to build with D-Bus support (newer version needed)

[Tim Van Holder <tim.vanholder@gmail.com>]

Current CVS emacs fails to build on my (admittedly outdated) Debian system:

dbusbind.o: In function `xd_remove_watch':
/home/tim/gnu/src/emacs/src/dbusbind.c:766: undefined reference to `dbus_watch_get_unix_fd'
/home/tim/gnu/src/emacs/src/dbusbind.c:768: undefined reference to `dbus_watch_get_socket'
dbusbind.o: In function `xd_add_watch':
/home/tim/gnu/src/emacs/src/dbusbind.c:741: undefined reference to `dbus_watch_get_unix_fd'
/home/tim/gnu/src/emacs/src/dbusbind.c:743: undefined reference to `dbus_watch_get_socket'

My guess is those are from a newer version of D-Bus; configure finds
version 1.x on my system:

checking for pkg-config... (cached) /usr/bin/pkg-config
checking for dbus-1 >= 1.0... yes
checking DBUS_CFLAGS... -I/usr/include/dbus-1.0 -I/usr/lib/dbus-1.0/include
checking DBUS_LIBS... -ldbus-1
checking for pkg-config... (cached) /usr/bin/pkg-config

So either those functions should only be used if a newer d-bus is found,
or, if they're needed, configure should look for the minimum version
required (and either error out or just disable dbus support).

Building using --without-dbus works fine of course, and I'm not sure I
even ever used D-Bus support (not consciously anyway); but the build
still should not fail (especially since I didn't actively enable the
dbus support in the first place).