setup: the mac-exe build (using py2app) requires macholib>=1.2.

Here is someone else's description of this problem:
http://rtmpy.org/ticket/7

mac/setup.py

@@ -45,7 +45,7 @@ setup_args = {
     'app': [ 'allmydata_tahoe.py' ],
     'options': { 'py2app': py2app_options },
     'data_files': data_files,
-    'setup_requires': [ 'py2app' ],
+    'setup_requires': [ 'py2app', 'macholib>=1.2' ],
     'packages': packages,
 }