Makefile: Generate dependency information implicitly while compiling

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.)