dbus service file: use substitution variable

The subsitution variable is needed for installing in other prefixes
than /usr. Otherwise, the dbus service file won't point to the right
installation directory.

Change-Id: If46688c9963cd69a931ffa4f6efc43cccbcff90c
Tuleap: #594

bin/dbus/cx.ring.Ring.service.in

 [D-BUS Service]
 Name=cx.ring.Ring
-Exec=/usr/sbin/dring
+Exec=@SBINDIR@/dring

configure.ac

@@ -366,6 +366,9 @@ AC_ARG_WITH([dbus],
 AS_IF([test "x$with_dbus" = "xyes"], [
        PKG_CHECK_MODULES(DBUSCPP, dbus-c++-1,, AC_MSG_WARN([Missing dbus development files]))
 
+       AS_AC_EXPAND(SBINDIR, $sbindir)
+       AC_SUBST(SBINDIR)
+
        AC_CONFIG_FILES([bin/dbus/Makefile
                         bin/dbus/cx.ring.Ring.service])