Allow WITH_JIT to be overridden in a custom buildspec.mk file.

Change-Id: I0fb7ffcfb9786f611e981d8f22f67a5ac8c9c405
diff --git a/core/combo/linux-arm.mk b/core/combo/linux-arm.mk
index 44d3540..c826831 100644
--- a/core/combo/linux-arm.mk
+++ b/core/combo/linux-arm.mk
@@ -198,8 +198,10 @@
 
 $(combo_target)CUSTOM_LD_COMMAND := true
 
-# Enable the Dalvik JIT compiler
-WITH_JIT := true
+# Enable the Dalvik JIT compiler if not already specified.
+ifeq ($(strip $(WITH_JIT)),)
+    WITH_JIT := true
+endif
 
 define transform-o-to-shared-lib-inner
 $(TARGET_CXX) \