Warn when a package uses TH but doesn't tell about it to Cabal

It seems like there are a lot of packages that use Template Haskell but don't tell Cabal about it via `extensions` or `other-extensions`. With GHC 7.8 (and probably also older ones) this can lead to build failures (see e.g. #1553). It'd be nice if we could do a check akin to #1455 and warn the user if this is the case.

This feature will likely require using GHC API, so perhaps we could combine it with #1455 - what we need is a `ghc --make` augmented to dump the dependency graph and the set of all enabled extensions before the actual compilation.

**Update:** see also #3081. If we implement this feature for TH, it will be easy to generalise for all extensions.