When i've tried to emerge mkvtoolnix, libboost was emerged successfully (as dependency), but mkvtoolnix build failed during configuration step with following message: checking for main in -lboost_regex... no checking for main in -lboost_regex-boost_regex... no configure: error: Could not link against boost_regex-boost_regex ! Then i did "eselect boost set 1", after that mkvtoolnix emerged successfully. So i guess, that "eselect" step should be added to libboost ebuild
There's already such step - it's activated by 'eselect' useflag, which is on by default. More likely is that something went wrong for you during boost upgrade/unmerge.
