Mac build fixes.

This gets us pretty close. There are a few problems with libraries we depend
on (system/core/liblog and external/icu4c) that I have workarounds for, and
a problem with gtest that I haven't yet worked around that prevents us from
linking any of the tests. But this at least gives us a Mac dex2oat binary.

Change-Id: Iac39a6c2963c3d37ab2165d7d1a70e303ba22c45
diff --git a/build/Android.libarttest.mk b/build/Android.libarttest.mk
index 86927c3..16bbcad 100644
--- a/build/Android.libarttest.mk
+++ b/build/Android.libarttest.mk
@@ -42,7 +42,10 @@
     include $(BUILD_SHARED_LIBRARY)
   else # host
     LOCAL_CFLAGS := $(ART_HOST_CFLAGS) $(ART_HOST_DEBUG_CFLAGS)
-    LOCAL_LDLIBS := -ldl -lrt -lpthread
+    LOCAL_LDLIBS := -ldl -lpthread
+    ifeq ($(HOST_OS),linux)
+      LOCAL_LDLIBS += -lrt
+    endif
     include $(BUILD_HOST_SHARED_LIBRARY)
   endif
 endef