boost::make_shared doesn't use constructor forwarding through move emulation

[me@…]

Currently, boost::make_shared doesn't perform constructor forwarding unless C++11 r-value references are supported. This prevents using of MOVABLE_BUT_NOT_COPYABLE types as arguments to constructors.

The Boost.Move documentation describes how to perform (limited) constructor forwarding. It's example is perfectly applicable to boost::make_shared.

The change in ​boostorg/smart_ptr#24 implements constructor forwarding for make_shared as documented in Boost.Move's documentation.

With this change the difference between the code supporting r-value references but not supporting variadic templates became the type signature and the call to boost::detail::sp_forward and boost::forward. The first difference was eliminated by using the macro BOOST_FWD_REF, the second by using boost::forward in both places, making the two pieces of code textually equal and thus I also removed the duplicate.

[me@…]

patch boostorg/smart_ptr#24 adding (partial) constructor forwarding on C++03