I'd suggest we consider to treat some items like Boost/Python/zlib in the same way like FFT:
even if they are not part of the toolchain formally, there are advantages to think of them as such in order to increase chances of combining modules together (in fact, we will need eventually in bioinformatics to produce such meta-modules). For the record: also I was doing it originally as you did (ie. prefer latest), only to find out that this actually makes more tricky the feature of combining items together...