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/tools/zipalign/Android.mk b/tools/zipalign/Android.mk
index 4194f81..cba6db3 100644
--- a/tools/zipalign/Android.mk
+++ b/tools/zipalign/Android.mk
@@ -22,15 +22,11 @@
 	liblog \
 	libzopfli
 
-ifeq ($(HOST_OS),linux)
-LOCAL_LDLIBS += -lrt
-endif
+LOCAL_LDLIBS_linux += -lrt
 
-ifdef USE_MINGW
-LOCAL_STATIC_LIBRARIES += libz
-else
-LOCAL_LDLIBS += -lz
-endif
+LOCAL_STATIC_LIBRARIES_windows += libz
+LOCAL_LDLIBS_linux += -lz
+LOCAL_LDLIBS_darwin += -lz
 
 ifneq ($(strip $(BUILD_HOST_static)),)
 LOCAL_LDLIBS += -lpthread