Fix compilation issues with GCC 8

src/graph/clustering/graph_extended_clustering.cc

src/graph/draw/graph_cairo_draw.cc

src/graph/generation/graph_community_network.hh

src/graph/gml.hh

src/graph/graph_io.cc

src/graph/graph_properties_copy.hh

src/graph/graph_reverse.hh

src/graph/inference/support/graph_state.hh

src/graph/topology/graph_all_distances.cc