Add _(darwin|linux|windows)

So that we can support building both linux and windows binaries at the
same time on a linux host. This replaces the ifeq($(HOST_OS),...) checks
in Android.mk files.

Bug: 23566667

Change-Id: I693e11984e36d55bb6f09fa0d49bc485463e16fb
diff --git a/libs/host/Android.mk b/libs/host/Android.mk
index 821f9b8..8c8f755 100644
--- a/libs/host/Android.mk
+++ b/libs/host/Android.mk
@@ -1,15 +1,10 @@
 LOCAL_PATH:= $(call my-dir)
 include $(CLEAR_VARS)
-LOCAL_ADDITIONAL_DEPENDENCIES := $(LOCAL_PATH)/Android.mk
 
 LOCAL_SRC_FILES:= \
     CopyFile.c
 
-ifeq ($(HOST_OS),darwin)
-LOCAL_CFLAGS += -DMACOSX_RSRC
-endif
-ifeq ($(HOST_OS),linux)
-endif
+LOCAL_CFLAGS_darwin += -DMACOSX_RSRC
 
 LOCAL_MODULE:= libhost
 LOCAL_C_INCLUDES := $(LOCAL_PATH)/include