Don't build ahat tests for mac.

Because they require running ART, which isn't supported on mac.

Change-Id: Ic4dea4a7cbbd8932694c0a2dd153e50e638ab171

Test: m ahat-test, but I can't test it fixes the mac locally.
diff --git a/tools/ahat/Android.mk b/tools/ahat/Android.mk
index f628fe5..4b3044a 100644
--- a/tools/ahat/Android.mk
+++ b/tools/ahat/Android.mk
@@ -40,6 +40,9 @@
 LOCAL_SRC_FILES := ahat
 include $(BUILD_PREBUILT)
 
+# The ahat tests rely on running ART to generate a heap dump for test, but ART
+# doesn't run on darwin. Only build and run the tests for linux.
+ifeq ($(HOST_OS),linux)
 # --- ahat-test-dump.jar --------------
 include $(CLEAR_VARS)
 LOCAL_MODULE := ahat-test-dump
@@ -102,6 +105,7 @@
 ahat-test: PRIVATE_AHAT_TEST_JAR := $(AHAT_TEST_JAR)
 ahat-test: $(AHAT_TEST_JAR)
 	java -enableassertions -jar $(PRIVATE_AHAT_TEST_JAR)
+endif # linux
 
 # Clean up local variables.
 AHAT_TEST_JAR :=