[gentoo-doc] Portage per-package environment/behavior

[Sven Vermeulen <sven.vermeulen@siphos.be>]

Hi guys,

I noticed we don't describe in the handbook that Portage can have
per-package environment variables (like CFLAGS) through /etc/portage/env.
This can be even (ab?)used to execute steps before or after specific phases
(based on the EBUILD_PHASE information), something I use for updating IDS
systems (postinst/prerm phase).

But I'm not sure if and where in the handbook this can be positioned best.
The environment variable stuff could be placed in the section on
"Environment Variables", but is quite off from the rest of the content
(since the rest of that chapter has nothing really to do with Portage or
build environments).

"Configuring through Variables" is probably the best location (somewhere in
the beginning as we talk there about Build-specific Options), but I do feel
that this particular feature is already more targeting advanced users, where
the location in the handbook somewhat suggests this for more beginner-like
types.

Perhaps another section in "Working with Portage", called "Advanced Portage
Features" or so? This can then contain the per-package env information, but
also overriding profile information and perhaps others we don't talk about
yet.

Any ideas on this?

	Sven