Backward compatibility fix to get this to compile on Max versions 9 and below, which don't define the _M macro.

pandatool/src/maxegg/maxNodeTree.cxx

@@ -15,6 +15,11 @@
 
 #include "maxEgg.h"
 
+// 3dsmax versions below 9 don't define this.
+#ifndef _M
+#define _M(s) (s)
+#endif
+
 ////////////////////////////////////////////////////////////////////
 //     Function: MaxNodeTree::Constructor
 //       Access: Public
@@ -486,4 +491,4 @@ void MaxNodeTree::set_collision_tags(MaxNodeDesc *node_desc, EggGroup *egg_group
       }
     }
     return;
-}
+}