[boost] [iostreams] fix cw link errors

I've committed the attached patch to fix a couple of link failures of iostreams tests on cw. Two tests, instead of linking to the library, just compile in some source files. At the same time, the tests managed to set BOOST_IOSTREAMS_DYN_LINK -- therefore adding dllimport to all declarations. The compiled-in source had no dllexport. MSVC seems to be forgiving in this situation, but cw is not. I'm 95% this change will fix those two errors, but if somebody thinks the change is wrong -- shout. - Volodya