favor of a common interface. Regardless of the compiler's capabilities, the
result type of a Boost.Function function object that was declared void will
be "unused". This allows the result of a Boost.Function function object to
be passed as a parameter regardless of whether the function is declared as
returning void. It greatly simplifies the use of Boost.Function objects with
wrapper objects (i.e., when the side effects are important, but the result
isn't: consider binding and composition when calling a std::for_each loop)
[SVN r10491]