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.

test/boost_has_hash.ipp

@@ -10,23 +10,8 @@
 //  DESCRIPTION:   The C++ implementation provides the (SGI) hash_set
 //                 or hash_map classes.
 
-#if defined(__GLIBCXX__) || (defined(__GLIBCPP__) && __GLIBCPP__>=20020514) // GCC >= 3.1.0
-#  ifdef BOOST_NO_CXX11_STD_UNORDERED
-#    define BOOST_STD_EXTENSION_NAMESPACE __gnu_cxx
-#    define _BACKWARD_BACKWARD_WARNING_H 1 /* turn off warnings from the headers below */
-#    include <ext/hash_set>
-#    include <ext/hash_map>
-#  else
-     // If we have BOOST_NO_CXX11_STD_UNORDERED *not* defined, then we must
-     // not include the <ext/*> headers as they clash with the C++0x
-     // headers.  ie in any given translation unit we can include one
-     // or the other, but not both.
-#    define DISABLE_BOOST_HAS_HASH_TEST
-#  endif
-#else
-#include <hash_set>
-#include <hash_map>
-#endif
+#include BOOST_HASH_SET_HEADER
+#include BOOST_HASH_MAP_HEADER
 
 #ifndef BOOST_STD_EXTENSION_NAMESPACE
 #define BOOST_STD_EXTENSION_NAMESPACE std