Can't install packages with my inplace compiler

[Sven Panne]

Hmmm, is this cabal mess the reason for the problems with GHC head and
Cabal head on https://travis-ci.org/haskell-opengl/StateVar/jobs/39533455#L102?
I've brought up the problem in another thread, but there was no
conclusion. As it is, there seems to be no way to test things with GHC
head on Travis CI, which is really bad. :-/ What can be done here?