Added option to pass custom C postprocessor

[czgdp1807]

cc: @certik

[certik]

I think as part of this change, let's not do --show-c, but rather save the result to a file ourselves, so no need for ">" which is quite fragile in build systems.

[czgdp1807]

I agree. Will do this on Thursday.

[czgdp1807]

Done. Please check the latest diff and let me know if you have any questions.

[certik]

Thanks for this, the other thing we need is to allow passing arguments to the script. I have just refactored such a script so that it accepts an argument of a some_file.inc that gets included at the top of the generated file as a header, instead of heaving the header hardwired in the script. I also need to add another argument with a file with a list of functions to modify.

[czgdp1807]

Well what do you think about this - Instead of passing a bunch of arguments to the post processor via LPython, can we just create a json file specifying all the configuration and the post processor can read this json file and do the necessary things. We can pass the path to this json file to the post processor via command line argument. So LPython will only call, python post_processor.py path_to_json_config_file and not python post_processor.py some_headers some_other_information some_more_information.

[certik]

Yes, the JSON approach is fine.

[certik]

I will test this PR manually and see if it satisfies our use case.