Documentation doesn't describe how to build with meson/ninja, only autotools.
I checked out gstreamer and some other subprojects from git for the first time and since they all seemed to be normal autotools projects, I built them with autogen.sh, configure and make.
There's no mention of how to build/install with meson in the README, the website, or the FAQ - all refer extensively to autotools. This is the first project I've ever encountered using meson so I had no idea about it - and on checking the source out from git there was nothing to indicate this was anything other than a purely autotools-based project. If you're retiring autotools in the near future then someone really needs to start thinking about documenting this.
Note that just linking to the meson & ninja documentation is not sufficient! When I looked these up myself the meson quick guide explained how to compile but not install - it then jumps straight ahead to distro packager use. The ninja manual is equally unhelpful for the end user; both projects' documentation is of course aimed at project authors.