DIP 11: trial partial implementation

[Adam D. Ruppe]

Jacob Carlborg wrote:
> * Run the compiler once with the -deps flag to collect the dependencies

-deps doesn't actually write anything unless the compile succeeds...
and it can't succeed unless you already know the dependencies!

The basic idea of caching the command might work though.
When I get sick of slowness, I manually copy/paste the command out
of the output and into a makefile.

Watching for failure to update the cache automatically ought to work
though, probably should do that.