Integrate dexpreopt into the build system.

Change-Id: Id67f85d0f5c8674f5bc22e431114ca73625811ef
diff --git a/core/Makefile b/core/Makefile
index ad964c1..f27b5c1 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -737,7 +737,8 @@
 
 ifdef WITH_DEXPREOPT
   ifndef DISABLE_DEXPREOPT
-    with_dexpreopt := true
+# TODO: remove the unnecessary code.
+#  with_dexpreopt := true
   endif
 endif
 ifdef with_dexpreopt