message: cxx.boost: Return copies of the configurations, to avoid modifications.
diff: @@ -235,6 +235,6 @@ for prop, library in Boost._Boost__libraries.items():
  setattr(self, name + '_header', Config(c))
  c.library_add(lib)
  setattr(self, name, Config(c))
  - return getattr(self, name + ('_header' if not link else ''))
  + return drake.cxx.Config(getattr(self, name + ('_header' if not link else '')))
  setattr(Boost, 'config_%s' % prop, config_getter)
  unclosure(prop, library)
  