[boost] MSVC warnings for BOOST_PP_IS_EMPTY in Boost.Phoenix

Damien Buhl wrote:

In all the test I ran I got no warning under VC++, but this was on a pretty new version. And some of my test were run before 1.57.

It probably depends on the VC++ version. 8.0 and 10.0 warn, 11.0 and 12.0 (I think) do not.