ci: Add build stage

So far we are only performing a basic syntax check on javascript
sources; it's time to test the C code as well. As mutter is tightly
coupled, we bite the bullet and build it as well, either using a
matching branch (if it exists), or current master.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/408

.gitlab-ci.yml

@@ -1,10 +1,17 @@
 stages:
  - review
  - source_check
+ - build
 
 variables:
     JS_LOG: "js-report.txt"
 
+.only_default: &only_default
+    only:
+        - branches
+        - tags
+        - merge_requests
+
 check_commit_log:
     image: registry.fedoraproject.org/fedora:latest
     stage: review
@@ -23,6 +30,7 @@ js_check:
     script:
         - find js -name '*.js' -exec js60 -c -s '{}' ';' 2>&1 | tee $JS_LOG
         - (! grep -q . $JS_LOG)
+    <<: *only_default
     only:
         changes:
             - js/**/*
@@ -30,6 +38,16 @@ js_check:
         paths:
             - ${JS_LOG}
         when: on_failure
-    only:
-        - merge_requests
-        - /^.*$/
+
+build:
+    image: registry.gitlab.gnome.org/gnome/gnome-shell/master:v1
+    stage: build
+    before_script:
+        - .gitlab-ci/checkout-mutter.sh
+        - meson mutter mutter/build --prefix=/usr -Dtests=false
+        - ninja -C mutter/build install
+    script:
+        - meson . build -Dbuiltype=debugoptimized
+        - ninja -C build
+        - ninja -C build install
+    <<: *only_default