gtkdoc rules do not depend on dependencies

`gnome.gtkdoc` accepts a `dependencies` keyword, but doesn't add those dependencies to the Ninja build rule. This means you need to remember to (re)build any targets needed before running any `-doc` targets.

Unfortunately, `gtkdoc` accepts internal and external dependencies, but custom targets don't seem to like those, so this is more complicated than I hoped to work on right now.

Start at the gnome.gtkdoc entry point and inspect how its dependencies are represented in the Ninja build rule, including the handling of internal and external dependencies for custom targets. Done means -doc targets trigger their declared dependencies without requiring a separate manual rebuild.

Written by the indexing model from the issue text.