D_BUSY added, to check recursive deps in def modules

lang/m2/comp/defmodule.c

@@ -55,7 +55,7 @@ GetDefinitionModule(id, incr)
 		We may have to read the definition module itself.
 		Also increment level by "incr".
 	*/
-	struct def *df;
+	register struct def *df;
 	static int level;
 	struct scopelist *vis;
 
@@ -101,6 +101,10 @@ GetDefinitionModule(id, incr)
 			df->mod_vis = vis;
 		}
 	}
+	else if (df->df_flags & D_BUSY) {
+		error("definition module \"%s\" depends on itself",
+			id->id_text);
+	}
 	else if (df == Defined && level == 1) {
 		error("cannot import from currently defined module");
 		df->df_kind = D_ERROR;