[gentoo-alt] Cross EPREFIX portage

[Markus Duft]

Hi!

Some time ago I talked on this list about my plans on using portage to
emerge packages for native Windows under Interix. To be able to do this,
haubi and I had to "help" portage a little :) we now have a patch ready with
which this is possible. Not only windows draws advantages out of the patch,
but now it is possible to create a "child" prefix. You can create a base
EPREFIX, which contains a system that doesn't change too often, and then
merge packages to different prefixes building upon that parent prefix.

For now, only DEPEND's are resolved from the parent EPREFIX -- RDEPEND and
PDEPEND *must* be installed in the child EPREFIX. If a DEPEND cannot be
merged into the parent prefix, portage tries to merge is to the child.

Maybe some of you can have a brief look at the patch, and if there are no
objections, I'd like to add the patch to the portage ebuild conditionally
with a USE flag for more testing. If it does not destroy anything (I greatly
doubt that it does...), it would be great to see it going into svn...

P.S.: haubi wrote up some comments on the patch and other things... I
attached them as-is, so ask haubi :)
P.S.S.: the patch is against anonsvn-checked-out trunk of prefix branch...

Cheers, Markus