last_value<void>::result_type set to last_value<void>::unusable when it could be set to void

[Stjepan Rajko]

Currently, last_value<void>::result_type is set to last_value<void>::unusable. On compilers which support void returns, it seems like it would be more beneficial to set it to void instead. I.e.,

template<> struct last_value<void> {

#ifdef BOOST_NO_VOID_RETURNS

struct unusable {};

public:

typedef unusable result_type;

#else

public:

typedef void result_type;

#endif BOOST_NO_VOID_RETURNS

...

Patch attached.