enable pkgconfig-depends for tests and benchmarks

This patch enables pkgconfig-depends for test suites and benchmarks.

Previously, this field was parsed but its contents were not applied to tests and benchmarks by the configure step.
On line 872, mapM is used to run addPkgConfigBILib on an optional library contained in a Maybe functor.

Previously, `updateLibrary` had type `Maybe Library -> IO (Maybe Library)` while `addPkgConfigBILib` has type `Library -> IO Library` as a result of my refactoring, which uses a common implementation of the conversion from pkgconfig dependencies to cc/ld flags.

Alternatively, I could remove the dependency on `Data.Traversable.mapM` by defining:

    addPkgConfigBILib Nothing = return Nothing
    addPkgConfigBILib (Just lib) = addPkgConfigBI ... ... lib