gitlab-ci: Use ccache

Meson picks it up automatically.

Based on:

* https://gitlab.freedesktop.org/mesa/mesa/merge_requests/240
* https://gould.cx/ted/blog/2017/06/10/ccache-for-Gitlab-CI/
* https://stackoverflow.com/questions/53659419/ccache-no-hits-in-gitlab-ci

v2 based on the corresponding Mesa change:
* Quote CCACHE_(BASE)DIR environment variables.
* Clear ccache stats in before_script.
* Move cache stanza to the build-and-test job, the cache isn't used in
  the docker-image job.

Reviewed-by: Adam Jackson <ajax@redhat.com> # v1

.gitlab-ci.yml

@@ -59,10 +59,19 @@ build-and-test:
         when: on_failure
         paths:
             - build/test/piglit-results/
+    cache:
+        paths:
+            - ccache/
     variables:
         LC_ALL: C.UTF-8
         PIGLIT_DIR: /root/piglit
         XTEST_DIR: /root/xts
+    before_script:
+        - export CCACHE_BASEDIR="$PWD"
+        - export CCACHE_DIR="$PWD/ccache"
+        - export CCACHE_COMPILERCHECK=content
+        - ccache --zero-stats
+        - ccache --show-stats
     script:
         - meson -Dprefix=/usr build/
         - |
@@ -74,3 +83,5 @@ build-and-test:
           cat build/meson-logs/testlog.txt
           cat build/test/piglit-results/xvfb/long-summary || :
           exit $status
+    after_script:
+        - CCACHE_DIR="$PWD/ccache" ccache --show-stats