uses nm options that arent supported by our nm

Index: tools/defcheck.py
--- tools/defcheck.py.orig
+++ tools/defcheck.py
@@ -47,6 +47,7 @@ def read_def_symbols(filename):
    return symbols
 
 have_errors = 0
+sys.exit (have_errors)
 
 
 #READ LIBRARY SYMBOLS
