win32 macro?

How to write code like this:
 
    bb_type_list := (if (
#ifdef Win32
Win32.FileSys.getFileAtrrivutes(U.replaceExt(dump_file_name,"bbtype")=NO
NE,
#else
OS.FileSys.access(U.replaceExt(dump_file_name,"bbtype"),
          [OS.FileSys.A_READ])
#endif) 
         then ReadInBBtypePairs (U.replaceExt(dump_file_name,"bbtype"))
         else []);

And which macro like Win32 can work in .cm
 
#ifdef Win32
win32.sml
#else
unix.sml
#endif
 
--william