code: def _startup(self):
          
          global _ac_config_logs
          global sconf_global
          global SConfFS
  
          self.lastEnvFs = self.env.fs
          self.env.fs = SConfFS
          self._createDir(self.confdir)
          self.confdir.up().add_ignore( [self.confdir] )
  
          if self.logfile is not None and not dryrun:
              # truncate logfile, if SConf.Configure is called for the first time
              # in a build
              if self.logfile in _ac_config_logs:
                  log_mode = "a"
              else:
                  _ac_config_logs[self.logfile] = None
                  log_mode = "w"
              fp = open(str(self.logfile), log_mode)
              self.logstream = SCons.Util.Unbuffered(fp)
              # logfile may stay in a build directory, so we tell
              # the build system not to override it with a eventually
              # existing file with the same name in the source directory
              self.logfile.dir.add_ignore( [self.logfile] )
  
              tb = traceback.extract_stack()[-3-self.depth]
              old_fs_dir = SConfFS.getcwd()
              SConfFS.chdir(SConfFS.Top, change_os_dir=0)
              self.logstream.write('file %s,line %d:\n\tConfigure(confdir = %s)\n' %
                                   (tb[0], tb[1], str(self.confdir)) )
              SConfFS.chdir(old_fs_dir)
          else:
              self.logstream = None
          # we use a special builder to create source files from TEXT
          action = SCons.Action.Action(_createSource,
                                       _stringSource)
          sconfSrcBld = SCons.Builder.Builder(action=action)
          self.env.Append( BUILDERS={'SConfSourceBuilder':sconfSrcBld} )
          self.config_h_text = _ac_config_hs.get(self.config_h, "")
          self.active = 1
          # only one SConf instance should be active at a time ...
          sconf_global = self
docs: Private method. Set up logstream, and set the environment
          variables necessary for a piped build