Add a gst-build subproject

This should help to reach 100% 'build from git'
status.

Fixes: #3795
12 jobs for gst-subproject in 37 minutes and 36 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  .Pre
passed style-check-diff #1225313

00:03:36

 
  Build
passed fedora-x86_64 #1225314

00:08:45

passed installed-tests #1225316

00:08:14

passed macos #1225318
macos

00:11:39

passed msys2-mingw64 #1225317
win32-ps

00:07:34

passed release-build #1225315

00:04:06

 
  Analysis
failed asan-build #1225320
asan allowed to fail

00:06:06

passed static-scan #1225319

00:37:35

 
  Docs
passed reference #1225321

00:06:45

 
  Flatpak
manual flatpak-manual:demo #1225322
flatpak allowed to fail manual
manual flatpak-manual:icon-browser #1225324
flatpak allowed to fail manual
manual flatpak-manual:widget-factory #1225323
flatpak allowed to fail manual
 
Name Stage Failure
failed
asan-build Analysis
 - css: 214 total, 212 pass, 2 fail
Processing gtk suite a11y:
- a11y: 22 total, 20 pass, 2 fail
Processing gtk suite tools:
- tools: 4 total, 3 pass, 1 fail
Processing gtk suite reftest:
- reftest: 134 total, 129 pass, 5 fail
Cleaning up file based variables
ERROR: Job failed: exit code 1