Propagate globally specified build-deps into each buildable component.

This is my attempt to fix #417.  Frankly, I'm not 100% sure I'm doing the right thing here, but it seems to have the right effect.  On the other hand, @dcoutts has announced that he was rewriting the parser of .cabal files using Parsec, so it's probably pointless to fix this issue now if the whole parser is going to be replaced.  I'm sending this pull request anyway.