On openSUSE 12.{1,2} and Fedora 17 libxml2 fails to compile due to gzopen64 not being defined. This comes from zlib, and hints at a mismatch between Z_LARGE64 and Z_WANT64.
I checked in a workaround, since it seems that bootstrapping is successful with the same code and without workaround in the emerge -e system phase.