Behaviour of inheriting cabal flags for cabal install

I find it very confusing that `cabal install --only-dependencies -fbuildBenchmark` also uses the `buildBenchmark` flag transitively for installing the dependencies.

I [came across this problem](https://github.com/JPMoresmau/eclipsefp/issues/104) when using EclipseFP. Two of my projects actually have the `buildBenchmark` flag and one requires GHC >=7.6 to build, the other GHC >=7.4. On my machine I only had GHC 7.4 installed and therefore made a tick in the EclipseFP GUI for the `buildBenchmark` flag only for the GHC 7.4 project. But when it installed its dependencies, the flag got passed over to the other library and then caused a build failure, as it required GHC 7.6.

If I understand it correctly, the current `-fflag` behaviour is "global" meaning that flags are used for all dependencies in a `cabal install` command. So, in order to not break backwards-compatibility why not introduce an option so that flags are only "local", that is, only valid for the immediate projects, and not its dependencies?