Disable turbine for unbundled builds

We don't yet have a turbine prebuilt, so disable turbine for
unbundled builds for now.

Test: none
Change-Id: I06179fca1764a86e387c8bd154ae052e2669d15b
diff --git a/core/combo/javac.mk b/core/combo/javac.mk
index 835ee82..b09b10c 100644
--- a/core/combo/javac.mk
+++ b/core/combo/javac.mk
@@ -22,6 +22,12 @@
     endif
 endif
 
+ifdef TARGET_BUILD_APPS
+  ifndef TURBINE_ENABLED
+    TURBINE_ENABLED := false
+  endif
+endif
+
 ifeq ($(OVERRIDE_ANDROID_JAVA_HOME),)
 ANDROID_JAVA_HOME := prebuilts/jdk/jdk8/$(HOST_PREBUILT_TAG)
 else