setup: work around old versions of distutils breaking setup.py

I'm not really sure how to trigger this, but we saw it in our build
environment for Windows at Google. This fixed it. Sigh.

setup.py

@@ -489,7 +489,11 b' class hgdist(Distribution):'
     negative_opt['no-rust'] = 'rust'
 
     def _set_command_options(self, command_obj, option_dict=None):
-        command_obj.boolean_options += self.boolean_options
+        # Not all distutils versions in the wild have boolean_options.
+        # This should be cleaned up when we're Python 3 only.
+        command_obj.boolean_options = (
+            getattr(command_obj, 'boolean_options', []) + self.boolean_options
+        )
         return Distribution._set_command_options(
             self, command_obj, option_dict=option_dict
         )