[Vala] generate deps file without first knowing dependencies?

[Jon Rosebaugh <chairos gmail com>]

Is there a way to generate the deps file (so it can be parsed for
dependencies of a given vala file) without first knowing the
dependencies and specifying them with --use-fast-vapi? It seems like
--deps can only be used when you're already doing a compilation of
some sort, and so that requires specifying all the dependencies with
--use-fast-vapi.

(Basically, I want to use https://github.com/apenwarr/redo to build,
instead of a Makefile. I've come up with a rule to automatically build
the fast-vapi from a .vala as required, but I still need to find a way
to determine which .vapi files are needed to compile a given .vala
down to C.)