Add entries for the weakref module to the build control.
diff --git a/setup.py b/setup.py
index 0e421c5..22866bb 100644
--- a/setup.py
+++ b/setup.py
@@ -153,6 +153,7 @@
         exts.append( Extension('regex', ['regexmodule.c', 'regexpr.c']) )
         exts.append( Extension('pcre', ['pcremodule.c', 'pypcre.c']) )
 
+        exts.append( Extension('_weakref', ['_weakref.c']) )
         exts.append( Extension('xreadlines', ['xreadlinesmodule.c']) )
 
         # array objects