code: def _have(self, name=None):
          
          if name is None:
              return (
                  (k, v) for k, v  in self.env.items()
                  if k.startswith('HAVE_')
              )
          return self.env.get('HAVE_' + self.env_key(name)) == True
docs: Check if a configure flag is set.
  
          If called without argument, it returns all HAVE_* items.
  
          Example:
  
              {% if have('netinet/ip.h') %}...{% endif %}