Add GitLab CI configuration

Builds the driver against all supported versions of xserver, with both
gcc and clang for xserver >= 1.18 (older versions cause warnings with
clang). Compiler warnings are treated as errors.

The xserver 1.15 build uses standalone glamor, the xserver 1.13 & 1.14
builds use --disable-glamor.

With the latest xserver version, make install and make distcheck are
tested as well.
11 jobs for gitlab-ci in 2 minutes and 8 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #26863
xserver-1.13

00:00:43

passed #26864
xserver-1.14

00:00:42

passed #26865
xserver-1.15

00:00:38

passed #26866
xserver-1.16

00:00:38

passed #26867
xserver-1.17

00:00:39

passed #26868
xserver-1.18

00:00:37

passed #26869
xserver-1.18-clang

00:00:40

passed #26870
xserver-1.19

00:00:35

passed #26871
xserver-1.19-clang

00:00:39

passed #26861
xserver-1.20

00:00:56

passed #26862
xserver-1.20-clang

00:00:43