Features relevance

[ghost]

At the moment, all properties are assumed to be relevant for target, and are encoded for path. For example, dvi files produced by tex might be placed in gcc/debug. Need to check which properties are really relevant for target. They include:

• properties that affect flags for the action
• properties that affect generators selection
• composite properties for which some part of expansion is relevant

The new functionality should be added to 'toolset' module and 'virtual-target.action' class, since the latter determines the path for targets.

A particularly nasty case is:

whatever a : a.cpp : <totally-irrelevant-feature>:<define>FOO ;

here, we better make the feature show up in target paths, since using it like this make it's actually relevant for this main target, even though it might be irrelevant for all the generators used.

[ghost]

Pushing later.