code: def has_pkgconfig(self):
          
          if self._has_pkgconfig is None:
              if self.use_pkgconfig:
                  line, exit_code = get_output("pkg-config igraph")
                  self._has_pkgconfig = (exit_code == 0)
              else:
                  self._has_pkgconfig = False
          return self._has_pkgconfig
docs: Returns whether ``pkg-config`` is available on the current system
          and it knows about igraph or not.