configure: Make dbus dir depend on the sysconf make variable

We want the directory path to be based on the sysconfdir as defined
during the make process. This way anyone overriding sysconfdir during
install won't have to also override the dbus directory.

Change-Id: Icfb8f381e29630592949abb411769ac1829002ae
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/configure.ac b/configure.ac
index 3e8773d..1d698ac 100644
--- a/configure.ac
+++ b/configure.ac
@@ -68,7 +68,7 @@
     AS_HELP_STRING([--with-dbuspolicydir=DIR], [Directory for D-Bus system policy files])
 )
 AS_IF([test "x$with_dbuspolicydir" = "x"],
-      [with_dbuspolicydir="${sysconfdir}/dbus-1/system.d"]
+      [with_dbuspolicydir='${sysconfdir}/dbus-1/system.d']
 )
 AC_SUBST([dbuspolicydir], [$with_dbuspolicydir])