Update libstdcpp3.hpp to use has_include to check for has_set/hash_map presence.

Correct doc typo.
Update tests to use BOOST_HASH_SET_HEADER.
Fixes https://github.com/boostorg/config/issues/349.

doc/html/boost_config/boost_macro_reference.html

@@ -1539,7 +1539,7 @@
                 <p>
                   The C++ implementation provides the (SGI) hash_set and hash_map
                   classes. When defined, <code class="computeroutput"><span class="identifier">BOOST_HASH_SET_HEADER</span></code>
-                  and <code class="computeroutput"><span class="identifier">BOOST_HASH_LIST_HEADER</span></code>
+                  and <code class="computeroutput"><span class="identifier">BOOST_HASH_MAP_HEADER</span></code>
                   will contain the names of the header needed to access hash_set
                   and hash_map; <code class="computeroutput"><span class="identifier">BOOST_STD_EXTENSION_NAMESPACE</span></code>
                   will provide the namespace in which the two class templates reside.

doc/html/index.html

@@ -990,7 +990,7 @@
 </div>
 </div>
 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
-<td align="left"><p><small>Last revised: July 27, 2020 at 17:18:39 GMT</small></p></td>
+<td align="left"><p><small>Last revised: December 18, 2020 at 10:47:00 GMT</small></p></td>
 <td align="right"><div class="copyright-footer"></div></td>
 </tr></table>
 <hr>