Warning from configure about ghc-prim.cabal

Running configure in a clean checkout, after checking for elfutils/libdw.h... I see

grep: libraries/ghc-prim/ghc-prim.cabal: No such file or directory

This started happening with fc186b0c, which made ghc-prim.cabal generated from ghc-prim.cabal.in during the hadrian build. Therefore configure shouldn't be looking at ghc-prim.cabal, it won't be there. cc @iliastsi