[boost] [1.34.0] iostream build fixes

Recently, Takeshi Mouri noticed that iostream library built does not include a couple of files, so not all functionality is available. It appears this went unnoticed for a long time, because iostreams tests (as early as 1.33.1) directly compiled in the necessary files, so tests worked but the iostreams library itself would not work for users. I've checked in, on HEAD, the attached patches. Thomas, what do you think about applying them to RC_1_34_0? I think they are relatively low risk, since the files in question are already compiled everywhere, and the patch only makes them included in a library. - Volodya