Proper dynamic linking on Windows

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Need decide how BOOST_ALL_DYN_LINK or BOOST_ALL_SOMELIB_LINK should be set when linking to a library using V2.

Issues:

1. If we're using V2 to link to a library, we don't need auto-linking, that's just extra work for the linker as we're linking to the library on the command line.
2. Without BOOST_ALL_DYN_LINK, Boost library headers won't contain necessary declexport declaration, so linking to DLL won't work.
3. It's easy to forget BOOST_ALL_DYN_LINK in client code.

I think the right solution would be for every library to have two extra elements in usage requirements:

• <link>shared:<define>BOOST_SOMELIB_PLEASE_USE_DECLSPEC_IN_HEADERS
• <link>shared:<define>BOOST_SUPPRESSAUTOLINK

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No problems, in practice.