Makefile: convenience targets to show dependencies and dependents

I often find myself wanting a fully expanded list of dependents to see what packages are affected and if I can exclude them from the build. A concrete example would be glib on FreeBSD, I have a large list of 'pkg_BUILD=' to see if everything else works without it. 

There's probably easier ways to this in other languages, but I thought it might be an idea to let Make do it's own dependency resolution by doing a fake build and changing one of the prerequisites.

It works well enough, other than OSX has a weird fsync that can be tricked by repeated calls. I tried doing this entirely with phony targets and not touching the filesystem, but my make-fu isn't up to it. 

Comments welcome.

Update: Another use would be to build all packages that depend on a certain package:
```
make `make show-dependents-qtbase | tr '\n' ' '`
```