Checking dependencies with Distribution

Hi, Can you help me understand how to check C library dependencies using Distribution.*? So far, reading Setup.hs from HDBC-mysql gave me a quick start. I believe what I need to do is to write a 'preConf' user hook. This preConf would probably mimic the behavior of autoconf macros provided by the library. Say, if that macro opens an include file and checks a string to confirm the version, that is what my preConf should do. But then I'm lost. Although my preConf probably doesn't need to add any information to the build process, I can't find how to actually break it if the proper version wasn't found, or how to emit a message saying "you need a newer version of library X". Thanks, Maurício