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)