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.