Add GitLab CI pipeline

Based on xf86-video-amdgpu, but applying experience gained in the
meantime in other projects and taking advantage of new features
available with current versions of GitLab.
12 jobs for gitlab-ci in 9 minutes and 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #9619327
container-build

00:08:40

 
  Build
passed #9619334
build-clang: [1.18]

00:00:42

passed #9619335
build-clang: [1.19]

00:00:49

passed #9619333
build-clang: [1.20]

00:00:49

passed #9619328
build-distcheck

00:01:04

passed #9619329
build-gcc: [1.16]

00:00:49

passed #9619330
build-gcc: [1.17]

00:00:47

passed #9619331
build-gcc: [1.18]

00:00:48

passed #9619332
build-gcc: [1.19]

00:00:47

passed #9619336
build-noglamor: [1.13]

00:00:38

passed #9619337
build-noglamor: [1.14]

00:00:47

passed #9619338
build-noglamor: [1.15]

00:00:37