This change is hopefully going to remedy evaluation errors like
at `haskellPackages_ghc721.hsdns' [system = "i686-darwin"]:
user-thrown exception: cannot bootstrap GHC on this platform
on Hydra. These errors occur because we build all packages with (almost)
all compilers an all "Haskell platforms". Some compilers, however,
cannot be bootstrapped on Darwin, so we ended up requesting builds that
cannot be performed.
svn path=/nixpkgs/trunk/; revision=28410
