uses nm options that arent supported by our nm

Index: tools/defcheck.py
--- tools/defcheck.py.orig
+++ tools/defcheck.py
@@ -40,6 +40,7 @@ exclude_symbols = [
 ]
 
 have_errors = 0
+sys.exit (have_errors)
 
 for df in def_files:
    directory, name = path.split (df)
