What are hookedPrograms for?

I'm looking at an example Duncan gave for Takusen's setup + .cabal: http://hpaste.org/6279 ... and do not understamd what hookedPrograms is all about. Can someone enlighten me? FWIW, my goal is to either locate or run some programs during the configure stage, and use the results (locating a program gives a directory, running a program gives directories printed on stdout) to modify the extraLibDirs and includeDirs in BuildInfo. I've been browsing the 1.3.12 sources, but no epiphany yet... Alistair