As pointed out by zooko.  For a use case, see issue842 where I ask Zooko to pull
something out of his brain for something that happened ages ago.

Since we target cabal now as primary, I'd vote for including flag values in 
there (there's little other variability with cabal... maybe curl version and 
such, but that would probably be a separate feature). That could be even doable 
in Setup.lhs as things are.