This updates the dependecy information on each successive recompile.

When building with MSVC, dependency information is generated with
a separate command just like before, but done together with
compiling each object file. (This is quite similar to how ffmpeg does
the same.)

This avoids the serial dependency generation step. In slow
environments (in particular if using MSVC) it could take a notable
amount of time; this can now all be done in parallel.

In one example, this reduces the time for a full build from clean
with MSVC (wrapped in wine) from 23 seconds down to 9 seconds,
thanks to parallelism. (For non-parallel builds, it doesn't make
much of a difference.)