Add HOST_CROSS_OS

Instead of using recursive make to change the HOST_OS when building the
windows SDK under linux, add the concept of cross-building to another
host os.

Bug: 23566667
Change-Id: I6dc525b601b6251d458d197c30bf4660d7485502
diff --git a/core/cxx_stl_setup.mk b/core/cxx_stl_setup.mk
index cda45a0..72e91e7 100644
--- a/core/cxx_stl_setup.mk
+++ b/core/cxx_stl_setup.mk
@@ -1,6 +1,6 @@
 #############################################################
 ## Set up flags based on LOCAL_CXX_STL.
-## Input variables: LOCAL_CXX_STL
+## Input variables: LOCAL_CXX_STL, my_prefix
 ## Output variables: My_cflags, my_c_includes, my_shared_libraries, etc.
 #############################################################