Thanks for the answers, all. :-) Since this doesn't seem to be the common knowledge I expected it to be, I'll try to work out how it's done properly and blog about it. And hopefully generate some much needed documentation for cabal along the way.
What I want specifically is to have happy produce a GLR parser from my .ly file, and I want this to happen during 'cabal install'. Which in turn means I want cabal to pass the "--glr" flag to happy during setup. My best guess is that I might want to use 'ppHappy' [1], or something in the vicinity, but there's no documentation for the combinator and it's far from obvious how to pass arguments to it.
I think the "right solution" is for the happy source file to specify what kind of grammar it is / should be produced. Yes, that would mean modifying happy.
Hmm, I agree with you in this particular case, regarding the grammar, since it really only makes sense for any given happy grammar to generate either kind of parser. But such a solution still wouldn't answer the general question about passing flags to preprocessors. Certainly not all such use cases are best fixed by pushing the flags into the pre-processed files. Cheers, /Niklas