gistfile1.txt

λ include → λ git master* → cabal configure;cabal build
Resolving dependencies...
Configuring hgit2-0.1...
Preprocessing library hgit2-0.1...
i686-apple-darwin11-llvm-gcc-4.2: warning: '-x c' after last input file has no effect
i686-apple-darwin11-llvm-gcc-4.2: no input files
c2hs: Error during preprocessing custom header file


Library
  HS-Source-Dirs:      src
  Exposed-Modules:     Data.HGit2.Blob, Data.HGit2.Commit, Data.HGit2.Config,
                       Data.HGit2.Errors, Data.HGit2.Git2, Data.HGit2.Index,
                       Data.HGit2.Object, Data.HGit2.Repository
  Includes:            git2.h
  Build-Tools:         c2hs
  CC-Options:          -U __BLOCKS__



It does work with this: cd src && ghc --make Main.hs -lgit2
(git2.h is in the same dir as the Cabal file)