Remove unnecessary includes of boost/bind.hpp, replace standard function wrappers.

boost/bind.hpp generates deprecation warnings about global placeholder argument
keywords. Removes the unnecessary includes of it.

Also, this commit replaces standard function object wrappers, which were removed
from C++20, with boost::bind.

Author: Lastique

example/options_heirarchy.cpp

@@ -21,12 +21,14 @@
 //
 
 #include <boost/program_options.hpp>
-namespace po = boost::program_options;
 #include <string>
 #include <iostream>
 #include <map>
 #include <stdexcept>
 #include <fstream>
+#include <boost/bind/bind.hpp>
+
+namespace po = boost::program_options;
 
 const std::string version("1.0");
 
@@ -221,7 +223,7 @@ class OptionsHeirarchy
          // The next two lines are the crazy syntax to use EnvironmentMapper as
          // the lookup function for env->config name conversions
          boost::function1<std::string, std::string>(
-         std::bind1st(std::mem_fun(&OptionsHeirarchy::EnvironmentMapper), this))),
+         boost::bind(&OptionsHeirarchy::EnvironmentMapper, this))),
          results);
       notify(results);
    }

test/test_convert.cpp

@@ -13,8 +13,6 @@
 #include <sstream>
 #include <iostream>
 #include <boost/progress.hpp>
-#include <boost/bind.hpp>
-#include <boost/ref.hpp>
 
 #include <boost/program_options/detail/convert.hpp>
 #include <boost/program_options/detail/utf8_codecvt_facet.hpp>