multiprecision, can convert from cpp_integer to void*

[Stepan Podoskin <stepik-777@…>]

MSVC 2010 is able to compile this:

#include <boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;

int main()
{
    cpp_int a, b;
    void* p = a*b;
    return 0;
}

GCC 4.7.2 can not compile this code, but it can compile this:

#include <boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;

int main()
{
    cpp_int a, b, c;
    void (*p)() = a*b;
    return 0;
}

[John Maddock]

(In [83162]) Change boolean context conversion operator so it's not convertible to void*. Fixes #8177.