localhost ~ # emerge -NDu world
Calculating world dependencies... done!
>>> Emerging (1 of 54) net-dns/libidn-0.5.15 to /
>>> checking ebuild checksums ;-)
>>> checking auxfile checksums ;-)
>>> checking miscfile checksums ;-)
>>> checking libidn-0.5.15.tar.gz ;-)
 * Enabling generation-2 compatibility ...
 * There was a problem determining which VM to use for generation-1
 * You may need to set your generation-1 VM again, and run env-update && source/etc/profile
 * Also, make sure you have followed the Java Upgrade Guide:
 * http://www.gentoo.org/proj/en/java/java-upgrade.xml                     [ !! ]

!!! ERROR: net-dns/libidn-0.5.15 failed.
Call stack:
  ebuild.sh, line 1555:   Called dyn_setup
  ebuild.sh, line 668:   Called pkg_setup
  ebuild.sh, line 1248:   Called java-pkg_pkg_setup
  java-pkg.eclass, line 41:   Called die

!!! Couldn't determine VM for generation-1
!!! If you need support, post the topmost build error, and the call stack if relevant.