use lowlevel os._exit() after forking

buildout.cfg

@@ -35,6 +35,6 @@ defaults =
 
 # python2.4 compatibility
 [versions]
-zope.interface = 3.7.0
+zope.interface = 3.6.7
 zope.exceptions = 3.7.1
 zope.testrunner = 4.0.1

src/zc/buildout/buildout.py

@@ -989,7 +989,7 @@ class Buildout(UserDict.DictMixin):
         env['PYTHONPATH'] = partsdir
         # windows: Popen will quote args itself if needed
         # see subprocess.list2cmdline
-        sys.exit(subprocess.Popen(args, env=env).wait())
+        os._exit(subprocess.Popen(args, env=env).wait())
 
     def _load_extensions(self):
         __doing__ = 'Loading extensions.'