Add MinGW autotools build jobs to CI

This is a draft since I've struggled for quite some time to get the jobs to pass the GitLab CI in less than the allowed 1 h run time that the shared Windows has hard-coded (cannot be changed by the user), without succeeding. Since there's been some interest from users for MinGW lately (!2236 (closed), !2289 (closed) & !2290 (merged)), I thought I'd as well might submit a draft MR to get some comments on how to proceed.

1. Any ideas on how to speed up the jobs would be highly appreciated. I've tried different attempts to workaround this problem, including using Docker containers with pre-installed dependencies and using the GitLab CI cache to store MinGW packages, but I haven't gotten any of them to work. My next approach will be to split the configure and make step into separate jobs and pass the configured build system as an artifact between them.
2. Should we instead focus on getting this merged without enabling the CI jobs?
3. The commit series is quite long (even though each commit is very small), 45 commits ATM, including the CI jobs. Is it to much to review in one go? I suspect so. Also, the last 6 commits need to be moved earlier in the commit series. I just haven't had the time to work on this for a while.

See also: - https://forum.graphviz.org/t/gitlab-runtimes/868 - !2289 (comment 737625744)