Add an osinclude.h below each OS directory, redirects by platform macro

* Linux folder has been renamed to Unix, to match defines and so that it
  compiles on OS X.
* This removes the need for a per-platform include search path for the
  right OS folder

StandAlone/StandAlone.cpp

@@ -49,7 +49,7 @@
 #include <stdlib.h>
 #include <math.h>
 
-#include "osinclude.h"
+#include "../glslang/OSDependent/osinclude.h"
 
 extern "C" {
     SH_IMPORT_EXPORT void ShOutputHtml();