The ebuilds for now-current sys-fs/zfs-0.6.0_rc9-r6 and sys-fs/zfs-9999 have a broken option in src_configure. Don't have a patch immediately handy, but "--with-udevdir" needs to have "udevdir udev" exec-quoted: --with-udevdir="$(tc-getPKG_CONFIG) --variable=$(udevdir udev)" as opposed to the current: --with-udevdir="$(tc-getPKG_CONFIG) --variable=udevdir udev" The latter fails during the install phase.
Actual failure log: /bin/mkdir -p '/var/tmp/portage/sys-fs/zfs-0.6.0_rc9-r6/image/pkg-config --variable=udevdir udev' /bin/sh ../../libtool --silent --mode=install /usr/bin/install -c zvol_id '/var/tmp/portage/sys-fs/zfs-0.6.0_rc9-r6/image/pkg-config --variable=udevdir udev' /usr/bin/install: unrecognized option '--variable=udevdir'
And I need to update that again - I misread the message and assumed the wrong fix. Looking at other samples from the tree, this would be a more appropriate fix: --with-udevdir="$($(tc-getPKG_CONFIG) --variable=udevdir udev)"
Thankyou for reporting this. This is now fixed in the tree. The fix should be available on mirrors within a few hours.