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From: Ruud Koolen <redlizard@gentoo.org>
To: gentoo-dev@lists.gentoo.org
Subject: [gentoo-dev] New globally masked prefix USE flag
Date: Tue, 26 Nov 2013 02:56:23 +0100
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Ohai,

As part of the RAP project[1], a version of Gentoo Prefix that uses its own 
libc rather than relying on the host version, prefix needs some new profiles 
serving this variant. While this is entirely inconsequential for most 
packages, there are a handful of ebuilds that need to distinguish between the 
two prefix variants.

The chief difference between the two variants from the point of view of 
ebuilds is that in classic prefix, the host /usr/include is a valid include 
path (as it contains libc headers), whereas in RAP prefix it is not, and 
analogously so for library paths; this needs special handling for packages 
with build scripts that do their own exiting custom library searching, such 
as python, which needs to do certain prefix-specific hacks only for classic 
prefix and not for RAP. 

To this end, I'm introducing a globally masked USE flag, "prefix-guest", which 
is masked in the base profile and unmasked and forced in some prefix 
profiles; this flag indicates that /usr/include, /lib, and /usr/lib are valid 
paths despite being on a prefix system. This flag will be on in classic 
prefix, and off in prefix RAP.

I will proceed in a week, assuming no complaints arise.

-- Ruud

[1] 
http://blogs.gentoo.org/news/2013/11/01/gentoo-monthly-newsletter-31-october-2013/#RAP