Minor fixes and a solution to the external-dependencies problem

Here we have some improvements that finally make parmake build *correctly* even when external packages change.

While some time ago I thought this would only be possible with external knowledge from cabal or similar (as implemented in my patch to [skip all Make-style time mchecks building when a SKIP_TIME_CHECK environment variable is set](https://github.com/nh2/ghc-parmake/commit/55678b15fa82bbb3ebc18dcca1e3662a7e26d793)), I recently found the [`ghc -M -include-pkg-deps`](http://www.haskell.org/ghc/docs/7.6.3/html/users_guide/separate-compilation.html#makefile-dependencies) option, that allows us to get external `.hi` dependencies as well.