Missing BOOST_NO_CXX11_HDR_SCOPED_ALLOCATOR

[pdimov]

We seem to be missing a macro for the C++11 header <scoped_allocator>.

[jzmaddock]

Is this still a concern or are we assuming C++11 as a base anyway now?

[pdimov]

Core libraries such as, well, Core, still support C++03 in principle. But I don't think we use <scoped_allocator> anywhere. The macro would only be for completeness.

[pdimov]

Actually we do use <scoped_allocator> in some Unordered tests (and a few other places), with bespoke checks for C++11 and GCC 4.8+. So the macro could have been used there were it available.