ghc-pkg refuses to register packages if some include directories don't exist
When registering a package, ghc-pkg fails with an error if any of the include directories do not exist. This is fine in general, but causes installation failures with gtk2hs. This is because the gtk2hs build tools will add all the directories given by "pkg-config --cflags gtk+-2.0", which typically contains lots of redundant and possibly non-existent directories, depending on your operating system.
Maybe ghc-pkg should just warn instead?