Compile the SGB examples. (#87)

Works fine with c++11, thanks!

System: (K)Ubuntu 16.04 LTS
Compiler: g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

Try to 'g++ -c' the four SGB examples girth.cpp, miles_span.cpp,
roget_components.cpp, and topo-sort-with-sgb.cpp.

(1) SGB 2002-01-30 changed the 'Associated Press scores' from 'ap0' and
'ap1' to 'ap'. This preprocessor macro (sic!) collides with numerous
function parameters in 'named_function_params.hpp'. '#undef'ining the
macro doesn't break the BGL examples.

(2) girth.cpp fails because of a missing 'null_vertex()' function.

(3) roget_components.cpp invokes 'strncmp()' from the C library.

(4) topo-sort-with-sgb.cpp hickups because of a missing 'vertices()'
function. Following roget_components.cpp and putting the collective
SGB header up front fixes this.

example/roget_components.cpp

@@ -7,7 +7,8 @@
 // http://www.boost.org/LICENSE_1_0.txt)
 //=======================================================================
 
-#include <stdio.h>
+#include <cstdio>
+#include <cstring>
 #include <iostream>
 #include <boost/graph/stanford_graph.hpp>
 #include <boost/graph/strong_components.hpp>

example/topo-sort-with-sgb.cpp

@@ -8,8 +8,8 @@
 #include <vector>
 #include <string>
 #include <iostream>
-#include <boost/graph/topological_sort.hpp>
 #include <boost/graph/stanford_graph.hpp>
+#include <boost/graph/topological_sort.hpp>
 
 int
 main()

include/boost/graph/stanford_graph.hpp

@@ -46,6 +46,8 @@ extern "C" {
 #include <gb_books.h> /* graphs based on literature */
 #include <gb_econ.h> /* graphs based on economic data */
 #include <gb_games.h> /* graphs based on football scores */
+#undef ap /* avoid name clash with BGL parameter */
+        // ap ==> Vertex::u.I
 #include <gb_gates.h> /* graphs based on logic circuits */
 #undef val /* avoid name clash with g++ headerfile stl_tempbuf.h */
         // val ==> Vertex::x.I
@@ -92,6 +94,9 @@ namespace boost {
     typedef directed_tag directed_category;
     typedef sgb_traversal_tag traversal_category;
     typedef allow_parallel_edge_tag edge_parallel_category;
+    /** Return a null descriptor */
+    static vertex_descriptor null_vertex()
+    { return NULL; }
   };
   template <> struct graph_traits<sgb_const_graph_ptr> {
     typedef Vertex* vertex_descriptor;
@@ -107,6 +112,9 @@ namespace boost {
     typedef directed_tag directed_category;
     typedef sgb_traversal_tag traversal_category;
     typedef allow_parallel_edge_tag edge_parallel_category;
+    /** Return a null descriptor */
+    static vertex_descriptor null_vertex()
+    { return NULL; }
   };
 }