Install meson from git when MESON_COMMIT variable is set

To help Meson project doing their QA when they do RC releases, it can be useful to run GStreamer CI using Meson from git. This could be automated with a curl command such as:

curl --globoff --request POST --header "PRIVATE-TOKEN: ******" "https://gitlab.freedesktop.org/api/v4/projects/gstreamer%2Fgst-ci/pipeline?ref=master&variables[][key]=MESON_COMMIT&variables[][value]=master" | jq ".web_url"

That command creates a pipeline in gst-ci master using meson master.

TODO:

• Do the same trick in .build windows before_script, but it needs powershell syntax I guess.
• Skip the manual step to launch all jobs.
• Is it possible to get ride of the token? Or have some public token that can only spawn a pipeline?
  • We don't want people to be able to anonymously spam us with pipelines. CI has a cost.
  • A trigger token can be created by any GStreamer dev, but cannot be public: https://docs.gitlab.com/ee/ci/triggers.
• Write a python script in Meson project that uses gitlab/github API to start CI in many projects and wait for result.
  • If we want to fully automate this, it would require someone to create an access token in each gitlab instance where there is a participating project.