При обновлении мира вышли колизии на порту dev-libs/boost-1.49.0-r2' NOT merged due to file collisions.

Всё просто:

What happens is this:
- you can’t use any version of boost older than 1.48 with GCC 4.7 or later;
- you can’t use any version of boost older than 1.50 with GLIBC 2.16;
- many packages don’t build properly with boost 1.50 and later;
- a handful of packages require boost 1.46;
- boost 1.50-r2 and later (in Gentoo) no longer support eselect boost making most of the packages using boost not build at all.