Use Jack by default

Change-Id: Ifc66fd863e3bfde780698f7bb4578927bcab4ff4
diff --git a/core/combo/javac.mk b/core/combo/javac.mk
index fb43ea4..82cbb43 100644
--- a/core/combo/javac.mk
+++ b/core/combo/javac.mk
@@ -9,6 +9,11 @@
 #   COMMON_JAVAC -- Java compiler command with common arguments
 #
 
+ifndef ANDROID_COMPILE_WITH_JACK
+# Defines if compilation with jack is enabled by default.
+ANDROID_COMPILE_WITH_JACK := true
+endif
+
 common_jdk_flags := -source 1.7 -target 1.7 -Xmaxerrs 9999999
 
 # Use the indexer wrapper to index the codebase instead of the javac compiler