gitlab-ci: Move meson job script to separate file

No functional change intended (except for no longer running meson
--version separately, as the version appears early in meson's output
anyway).

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 0feba19..1b448b3 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -79,31 +79,7 @@
 .meson-build:
   extends: .build
   script:
-    # We need to control the version of llvm-config we're using, so we'll
-    # generate a native file to do so. This requires meson >=0.49
-    - if test -n "$LLVM_VERSION"; then
-        LLVM_CONFIG="llvm-config-${LLVM_VERSION}";
-        echo -e "[binaries]\nllvm-config = '`which $LLVM_CONFIG`'" > native.file;
-        $LLVM_CONFIG --version;
-      else
-        touch native.file;
-      fi
-    - meson --version
-    - meson _build
-            --native-file=native.file
-            -D buildtype=debug
-            -D build-tests=true
-            -D libunwind=${UNWIND}
-            ${DRI_LOADERS}
-            -D dri-drivers=${DRI_DRIVERS:-[]}
-            ${GALLIUM_ST}
-            -D gallium-drivers=${GALLIUM_DRIVERS:-[]}
-            -D vulkan-drivers=${VULKAN_DRIVERS:-[]}
-            -D I-love-half-baked-turnips=true
-    - cd _build
-    - meson configure
-    - ninja -j4
-    - LC_ALL=C.UTF-8 ninja test
+    - .gitlab-ci/meson-build.sh
 
 .scons-build:
   extends: .build