Build external/jarjar only if no prebuilt is present.

Change-Id: I778c1c30ec299dc308ced9c791fa6ff9683712b2
diff --git a/Android.mk b/Android.mk
index 038076c..19da7a4 100644
--- a/Android.mk
+++ b/Android.mk
@@ -15,7 +15,7 @@
 LOCAL_PATH := $(call my-dir)
 include $(CLEAR_VARS)
 
-ifdef BUILD_JARJAR_FROM_SOURCE
+ifeq ($(wildcard prebuilts/misc/common/jarjar),)
 
 LOCAL_SRC_FILES := $(call all-java-files-under, src/main)