Draft: contrib: add list-videolan.mak

Add a dedicated makefile to list contribs downloaded via download_pkg that have not been uploaded to the FTP. It currently doesn't check every contrib because of how the contrib system is made, but it already checks a lot of contribs.

The behaviour can be overloaded through the following commandline:

make check-all -skf list-videolan.mak action_url='echo $(1) -> $(2)'

The parameters passed to action_url are:

• $(1): the upstream url to the contrib file
• $(2): where the videolan contrib is expected to be found

It's currently a work-in-progress script and it might change completely to provide more exhaustive checks, but I still create a merge request here as a baseline for discussion on how to proceed with the checks the best way possible.

Among the current limitation:

• it currently depends on bootstrap parameters, that are hardcoded and unused.
• it still create sums files before removing them (not sure we can do anything with that regards).
• it doesn't expose HAVE_foo variables, which is not an issue when only checking, but might be if, for example, the URL is conditionally defined.
• it's a bit primitive to use.

However, checking and having the maximum of contribs in the FTP will help mitigate CI failures and reduce the load, while ensuring that we can reproduce the compilation from past sources as much as possible. Having an automated way to check/update this will help reduce the cost of doing so.