As the title asks, “What is the platform macro for Linux?”
Unity - Manual: Conditional compilation that page doesn’t say anything about the Linux, but it would be reasonable assumption that the macro would be UNITY_STANDALONE_LIN since UNITY_STANDALONE_WIN is for Windows and UNITY_STANDALONE_OSX is for Mac OS X.
Unfortunately UNITY_STANDALONE_LIN doesn’t return any Google hits (well it will after this post, but anyways), and some hits (binrand dot com) are returned if search term is UNITY_STANDALONE_LINUX.