Merge topic 'drop-ancient-workarounds'

0f7bdd61 Remove VS 6 special case.
5e92c826 Remove some obsolete stuff.
15e42bb2 cmStandardIncludes: Remove obsolete cmOStringStream.
931e055d Port all cmOStringStream to std::ostringstream.
f194a009 Remove unused cmIStringStream class.
3ec1bb15 cmStandardIncludes: Remove std namespace hack.
bb3bce70 cmStandardIncludes: Remove ANSI_FOR_SCOPE hack.
28fa4923 cmStandardIncludes: Remove iostreams workaround for obsolete Compaq compiler.
837a8a63 cmStandardIncludes: Drop Comeau-related workaround.
4030ddfd Remove Borland-related undef.
17d6a6fd cmStandardIncludes: Remove comment about Borland.
26fb5011 Drop SGI as a CMake host compiler.