def use_built_igraph(self):
        """Assumes that igraph is built already in ``igraphcore`` and sets up
        the include and library paths and the library names accordingly."""
        buildcfg.include_dirs = [os.path.join("igraphcore", "include")]
        buildcfg.library_dirs = [os.path.join("igraphcore", "lib")]
        buildcfg.static_extension = True

        buildcfg_file = os.path.join("igraphcore", "build.cfg")
        if os.path.exists(buildcfg_file):
            buildcfg.libraries = eval(open(buildcfg_file).read())
Assumes that igraph is built already in ``igraphcore`` and sets up
        the include and library paths and the library names accordingly.