Created attachment 314007 [details] /usr/portage/log/sys-fs:zfs-9999:20120603-030629.log gonna attach logs
Created attachment 314009 [details] /var/tmp/portage/sys-fs/zfs-9999/work/zfs-9999/config.log
Created attachment 314011 [details] emerge --info
Created attachment 314013 [details] emerge --info gcc
A patch has been sent upstream for this: https://github.com/zfsonlinux/zfs/pull/767
sys-fs/zfs-0.6.0_rc9 is now in tree with the patch for this.
The patch is in portage and it appears that any lingering issues can be solved by doing "make clean" before building your kernel.