options cannot be stored in variables

It seems like storing options in variables doesn't work.

Sample code (based on tutorial):
```C++
#include
using namespace boost::program_options;

#include
using namespace std;

/* Auxiliary functions for checking input for validity. */

/* Function used to check that 'opt1' and 'opt2' are not specified
at the same time. */
void conflicting_options(const variables_map& vm,
const char* opt1, const char* opt2)
{
if (vm.count(opt1) && !vm[opt1].defaulted()
&& vm.count(opt2) && !vm[opt2].defaulted())
throw logic_error(string("Conflicting options '")
+ opt1 + "' and '" + opt2 + "'.");
}

/* Function used to check that of 'for_what' is specified, then
'required_option' is specified too. */
void option_dependency(const variables_map& vm,
const char* for_what, const char* required_option)
{
if (vm.count(for_what) && !vm[for_what].defaulted())
if (vm.count(required_option) == 0 || vm[required_option].defaulted())
throw logic_error(string("Option '") + for_what
+ "' requires option '" + required_option + "'.");
}

int main(int argc, char* argv[])
{
try {
string ofile;
string macrofile, libmakfile;
bool t_given = false;
bool b_given = false;
string mainpackage;
string depends = "deps_file";
string sources = "src_file";
string root = ".";

options_description desc("Allowed options");
desc.add_options()
// First parameter describes option name/short name
// The second is parameter to option
// The third is description
("help,h", "print usage message")
("output,o", value(&ofile), "pathname for output")
("macrofile,m", value(¯ofile), "full pathname of macro.h")
("two,t", bool_switch(&t_given), "preprocess both header and body")
("body,b", bool_switch(&b_given), "preprocess body in the header context")
("libmakfile,l", value(&libmakfile),
"write include makefile for library")
("mainpackage,p", value(&mainpackage),
"output dependency information")
("depends,d", value(&depends),
"write dependencies to ")
("sources,s", value(&sources), "write source package list to ")
("root,r", value(&root), "treat as project root directory")
;

variables_map vm;
store(parse_command_line(argc, argv, desc), vm);

if (vm.count("help")) {
cout << desc << "\n";
return 0;
}

conflicting_options(vm, "output", "two");
conflicting_options(vm, "output", "body");
conflicting_options(vm, "output", "mainpackage");
conflicting_options(vm, "two", "mainpackage");
conflicting_options(vm, "body", "mainpackage");

conflicting_options(vm, "two", "body");
conflicting_options(vm, "libmakfile", "mainpackage");
conflicting_options(vm, "libmakfile", "mainpackage");

option_dependency(vm, "depends", "mainpackage");
option_dependency(vm, "sources", "mainpackage");
option_dependency(vm, "root", "mainpackage");

cout << "two = " << vm["two"].as() << "\n";
cout << "depends = " << depends << "\n";
cout << "depends = " << vm["depends"].as() << "\n";
}
catch (exception& e) {
cerr << e.what() << "\n";
}
}
```

And here is an example console printout:
```
$ ./Gmame.exe -p e -d fg
two = 0
depends = deps_file
depends = fg
```

It seems like the provided storage variable is just being ignored? If this is incorrect somehow then the tutorial needs to be updated as well.

Building using boost 1.68 on Windows using MSVC 2017.

Inizia dal punto di ingresso principale fornito e riproduci l'esempio da riga di comando usando Boost 1.68 su MSVC 2017. Confronta le variabili associate, come depends, con i valori corrispondenti di variables_map e verifica le indicazioni del tutorial sulle variabili di archiviazione. Il lavoro è completato quando il comportamento è spiegato e corretto oppure documentato chiaramente, e l'esempio produce valori coerenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.