uses nm options that arent supported by our nm

Index: tools/defcheck.py
--- tools/defcheck.py.orig
+++ tools/defcheck.py
@@ -46,6 +46,7 @@ def_files = (
 )
 
 have_errors = 0
+sys.exit (have_errors)
 
 srcdir = None
 if len(sys.argv) > 1: