configure.ac: fix have_dbus if dbus support is disabled

If dbus support is explicitly disabled, $have_dbus should be no, but was
empty. systemd-login support depends on dbus, but the check does not
trigger correctly, if $have_dbus is empty.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

configure.ac

@@ -502,6 +502,7 @@ AC_ARG_ENABLE(dbus,
               AS_HELP_STRING([--disable-dbus],
                              [do not build with dbus support]),,
               enable_dbus=auto)
+have_dbus=no
 if test "x$enable_dbus" != "xno"; then
         PKG_CHECK_MODULES(DBUS,
                           dbus-1 >= 1.6,