cabal should tell hsc2hs where to find gcc

I'm not sure where things break down, but you can see that hsc2hs gets `gcc` from the PATH by doing something roughly like this:

```
touch ~/.cabal/bin/gcc; chmod a+x ~/.cabal/bin/gcc; hash -r
mkdir hsctest; cd hsctest
cabal init # make a library, but it doesn't matter
touch LICENSE
cat > Bar.hs < Foo.hsc <

Reproduce the issue with the hsctest.cabal project, Bar.hs, and Foo.hsc using the two cabal install commands. Start by tracing how hsc2hs locates gcc during the Foo.hsc build and compare that with ghc's behavior; done means a PATH entry named gcc cannot silently control hsc2hs when the compiler already identifies the toolchain.

Written by the indexing model from the issue text.