Issue with compiling the graph

[vinitkumar]

There is a crash when trying to compile. Looks like the way, the members are accessed is not correct.
Compiling on Mac gives a cryptic error, this is better.

/usr/bin/ld: t_graph.c:(.text+0x76): undefined reference to `graph_create'est_graph.c:(.text+0x86): undefined reference to `graph_vertex_count'
/usr/bin/ld: test_graph.c:(.text+0xd0): undefined reference to `graph_has_edge'
/usr/bin/ld: test_graph.c:(.text+0x128): undefined reference to `graph_out_degree'
/usr/bin/ld: test_graph.c:(.text+0x166): undefined reference to `graph_foreach'
/usr/bin/ld: test_graph.c:(.text+0x183): undefined reference to `graph_edge_count'
/usr/bin/ld: test_graph.c:(.text+0x1c3): undefined reference to `graph_add_edge'
/usr/bin/ld: test_graph.c:(.text+0x1e0): undefined reference to `graph_edge_count'
/usr/bin/ld: test_graph.c:(.text+0x22a): undefined reference to `graph_has_edge'
/usr/bin/ld: test_graph.c:(.text+0x28e): undefined reference to `graph_out_degree'
/usr/bin/ld: test_graph.c:(.text+0x2cf): undefined reference to `graph_foreach'
/usr/bin/ld: test_graph.c:(.text+0x30e): undefined reference to `graph_add_edge'
/usr/bin/ld: test_graph.c:(.text+0x335): undefined reference to `graph_edge_count'
/usr/bin/ld: test_graph.c:(.text+0x378): undefined reference to `graph_out_degree'
/usr/bin/ld: test_graph.c:(.text+0x3b9): undefined reference to `graph_has_edge'
/usr/bin/ld: test_graph.c:(.text+0x408): undefined reference to `graph_destroy'
collect2: error: ld returned 1 exit status