target oldest possible platform in default build

From bruen...@google.com on April 10, 2014 17:27:46

For our release packages to support Windows 2000 and Windows XP we are setting the header defines properly but we're not building with the right /subsystem flag. We should set it as old as possible by default for the compiler in use.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1424