Vppreproc: MMD feature

[piecea]

Could the -MMD option (like that of gcc, but also of verilator) have a place in vppreproc? Note –MMD generates a dependency output file for all the user files. We are interested in this feature for automating our compile flow.

If vppreproc wants to focus only on preprocessing, maybe a -dump-includes option is more suitable?

Using only the verilog api, we have prototyped an implementation of the -MMD option and can share the code. but, we think it would be useful to be provide this option natively in vppreproc.

[wsnyder]

I'd probably accept this if you want to make a pull request.

I would note you might alternatively use "verilator -MMD -E" which should already support this.