build: Require dbus for systemd-login

systemd-login support requires dbus (see "dbus.h" header in
"launcher-logind.c") but the configure script was only
checking libsystemd-login availability to define the
HAVE_SYSTEMD_LOGIN macro, which results in undefined
symbols in launcher-unit.

Put the systemd-login checks after the dbus ones, and only
run the checks if it is present. Also mention dbus in the
error message if "--enable-systemd-login" was forced.

Signed-off-by: Manuel Bachmann <manuel.bachmann@iot.bzh>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
1 file changed