What Cabal stuff is permanent?

I have been using the info at http://www.haskell.org/ghc/docs/latest/html/Cabal/authors.html as my guide to Cabal. I recently added an include-dirs line to my .cabal file, only to find that ghc-pkg would fail to register this package. It appears that include-dirs was placed into the ghc-pkg file, and of course, the relative path doesn't exist at install time and isn't even relevant there. The problem is: the Cabal docs don't say which fields are "permanent" and registered in the package and which aren't. There are many cases where it is not desirable to have fields be "permanent", and this is one such case. I'm not sure how to specify "just search for .h files here at build time of this library" through Cabal. Ideas? -- John